e  Silver  Bro 


•:arcn  .Labor 


Monographs  on  the  Theory  of  Photography  from  the 
Research  Laboratory  of  the  Eastman  Kodak  Co. 

No.  1 


COPYRIGHT  1921 
EASTMAN  KODAK  COMPANY 


The  Silver  Bromide  Grain 

of  Photographic 

Emulsions 


A.  P.  H.Trivelli  and 

S.  E.  Sheppard 


ILLUSTRATED 


D.  VAN  NOSTRAND  COMPANY 
NEW  YORK 

EASTMAN  KODAK  COMPANY 

ROCHESTER,  N.  Y. 
1921 


a- 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

Edited  by 
C.  E.  KENNETH  MEES 

and 
MILDRED  SPARGO  SCHRAMM 


Monographs  on  the  Theory 
of  Photography 

THE  SILVER  BROMIDE  GRAIN  or  PHOTOGRAPHIC  EMULSIONS. 
By  A.  P.  H.  Trivelli  and  S.  E.  Sheppard. 
Other  volumes  soon  to  appear. 

GELATINE.     By  S.  E.  Sheppard,  D.  Sc.     2  volumes. 
THE  THEORY  or  DEVELOPMENT.     By  A.  H.  Nietz. 


465033 


Preface  to  the  Series 


The  Research  Laboratory  of  the  Eastman 
Kodak  Company  was  founded  in  1913  .to  carry 
out  research  on  photography  and  on  the  pro- 
cesses of  photographic  manufacture. 

The  scientific  results  obtained  in  the  Labora- 
tory are  published  in  various  scientific  and  tech- 
nical journals,  but  the  work  on  the  theory  of 
photography  is  of  so  general  a  nature  and  occu- 
pies so  large  a  part  of  the  field  that  it  has  been 
thought  wise  to  prepare  a  series  of  monographs, 
of  which  this  volume  is  the  first.  In  the  course 
of  the  series  it  is  hoped  to  cover  the  entire  field 
of  scientific  photography,  and  thus  to  make 
available  to  the  general  public  material  which 
at  the  present  time  is  distributed  throughout  a 
wide  range  of  journals.  Each  monograph  is  in- 
tended to  be  complete  in  itself  and  to  cover,  not 
only  the  work  done  in  the  Laboratory,  but  also 
that  available  in  the  literature  of  the  subject. 
A  very  large  portion  of  the  material  in  these 
monographs,  however,  will  naturally  be  original 
work  which  has  not  previously  been  published. 
The  monographs  are  written  by  those  specialists 
in  the  Laboratory  who  are  best  qualified  for  the 
task,  each  monograph  being  edited  by  the  Direc- 
tor of  the  Laboratory  and  by  Mrs.  Schramm, 
who  is  the  active  editor  of  the  series. 

Rochester,  New  York 
April,  1921 


Preface 

The  fundamental  units  of  the  sensitive  materials  used  in 
photography  are  the  small  grains  of  silver  halide  which, 
imbedded  in  gelatine,  form  the  emulsion. 

Since  these  grains  are  of  very  small  size,  and  are,  further- 
more, precipitated  in  a  colloid  medium,  they  have  usually 
been  treated  simply  as  colloid  aggregates. 

As  a  result  of  a  complete  crystallographic  study  involving 
photomicrographic  work  of  a  high  order,  it  has  been  possible 
not  only  to  confirm  the  fact  that  the  grains  of  high-speed 
emulsions  are  definitely  crystalline,  but  to  identify  their 
crystalline  form  and  to  show  that  all  the  grains,  though  having 
several  distinct  shapes,  belong  to  the  same  crystalline  class. 
The  grains  being  thus  established  as  micro-crystalline,  their 
formation  in  the  emulsion  can  be  studied  by  the  aid  of  the 
recent  physico-chemical  theories  of  precipitation  and  especially 
of  the  dispersion  theory  of  Von  Weimarn,  according  to  which 
the  dispersity  of  the  initial  precipitate  will  be  determined  by 
the  concentration  of  the  solutions  and  other  physical  condi- 
tions. The  changes  in  the  dispersity  of  the  original  precipitate 
during  ripening,  which  will  follow  from  the  laws  of  surface 
energy,  is  now  found  to  be  related  to  changes  in  the  content  of 
adsorbed  impurities,  and  in  connection  with  this  the  effect 
of  ammonia  upon  the  grains  has  been  studied. 

The  catalysis  of  crystallization  by  nuclei  is  suggested  as 
an  explanation  of  some  of  the  effects  produced  by  the 
admixture  of  silver  iodide  with  silver  bromide  in  an  emul- 
sion, and  the  fact  that  traces  of  colloidal  silver  make  the 
grains  color-sensitive  is  believed  to  be  related  to  this. 

A  study  of  the  relations  existing  between  the  sizes  of  the 
grains  and  their  photographic  properties  is  reserved  for  a 
future  monograph. 

Rochester,  New  York 
April,  1921 


The  Silver  Bromide  Grain  of 
Photographic  Emulsions 


CONTENTS 


PREFACE 
CHAPTER 


I. 


CHAPTER   II 


CHAPTER  III. 


CHAPTER 
CHAPTER 
CHAPTER 


IV. 

V. 

VI. 


CHAPTER    VII. 
CHAPTER  VIII. 

CHAPTER     IX. 
CHAPTER       X. 


Page 

7 


The  influence  of  ammonia  on  photo- 
graphic emulsions  and  a  theory  of 
ripening 11 

Von  Weimarn's  theory  and  the  deter- 
mination of  the  dispersity  of  silver  bro- 
mide precipitates 27 

Accessory  factors  influencing  the  disper- 
sity of  silver  bromide  emulsions  .  .  35 

Crystallization  catalysis 52 

Capillarity  and  crystalline  growth   .      .      57 

Experimental  study  of  the  crystalli- 
zation of  silver  bromide 75 

The  classification  of  silver  halide  crystals      95 

The  silver  bromide  crystals  of  photo- 
graphic emulsions 99 

The  directions  of  most  rapid  growth  in 
silver  bromide  crystals,  and  the  occur- 
rence of  anomalous  forms  .  .  .  .107 

The  behavior  of  silver  bromide  and  sil- 
ver iodo-bromide  crystals  in  polarized 
light 121 

Summary  of  crystallographic  study  of  silver  halide  grains  .    129 

Alphabetical  list  of  serial  publications  referred  to,  with  the 
abbreviations  adopted  in  citations 132 

Bibliography 133 

Index  of  Authors 137 

Index  of  Subjects 139 


The  Silver  Bromide  Grain  of 
Photographic  Emulsions 

CHAPTER   I 

The  Influence  of  Ammonia  on  Photographic 
Emulsions  and  a  Theory  of  Ripening 

The  use  of  aqueous  ammonia  in  the  ripening  of  photographic 
silver  halide  emulsions  was  introduced  by  Johnston1  and  is 
well  known  to  photographic  technologists,  particularly  through 
the  later  work  of  J.  M.  Eder.2 

Eder  states  that  exposing  a  dry  gelatino-bromide  plate 
for  a  few  minutes  to  the  vapor  from  strong  ammonia  imme- 
diately before  using  in  the  camera  results  in  a  marked  increase 
in  sensitiveness.  On  the  other  hand,  Gaedicke3  concluded 
that  fuming  prior  to  exposure  diminished  the  sensitiveness, 
but  that,  subsequent  to  exposure  and  prior  to  development, 
it  increased  the  developability  of  the  latent  image,  resulting 
in  an  effective  sensitizing.  This  action  he  considered  to  be 
one  on  the  latent  image,  not  an  acceleration  of  the  action  of 
the  developer.  Sheppard  and  Mees4  found  that  certain 
plates  gave  a  higher  inertia,  or  lower  speed,  with  ferrous  oxalate 
developer  than  with  organic  developers,  while  a  larger  group 
gave  practically  the  same  speed  with  both  developers.  For 
the  latter,  however,  a  slight  fuming  with  ammonia  increased 
the  inertia,  i.  e.,  decreased  the  speed,  when  ferrous  oxalate 
was  used  as  developer.  In  addition  to  these  relatively  invisible 
effects,  the  accounts  of '  which  exhibit  rather  contradictory 
conclusions,  it  was  observed  by  Eder  that,  if  moist  silver 
bromide  plates  were  exposed  under  a  bell-jar  to  ammonia 
vapor  for  a  considerable  time,  they  became  more  sensitive  to 
light  and  coarser-grained,  ultimately  forming  a  network  of 
coarse-grained  silver  bromide  with  relatively  empty  inter- 
spaces resembling  frost  figures. 

1  Johnston,  J.,  Gelatino-bromide  of  silver  emulsions  treated  with  ammonia.     Brit.  J. 
Phot.  Almanac  1877:  95.   1877. 

2  Eder,  J.  M.,  Beitrage  zur  Photochemie  des  Bromsilbers.    Sitzungsber.  Akad.  Wiss. 
Wien.  81:  679.   1880. 

3  Gaedicke,  J.,  Ammoniakraucherung  bei  Trockenplatten.    Jahrb.  Phot.  27:  62.  1913. 

4  Sheppard,  S.  E.,  and  Mees,  C.  E.  K.,  Investigations  on  the  theory  of  the  photo- 
graphic process. 

11 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

Englisch1  found  that  a  partial  development  of  the  latent 
image  was  possible,  for  by  treating  an  exposed  plate  with 
strong  aqueous  ammonia  the  unexposed  parts  were  apparently 
more  rapidly  dissolved  away  than  the  exposed  parts.  He 
attributed  this  to  a  lesser  solubility  of  the  exposed  halide  in 
ammonia.  Luppo-Cramer,2  repeating  the  experiments,  came 
to  a  different  conclusion.  Under  suitable  experimental  con- 
ditions he  found  that  the  exposed  portions  at  first  apparently 
dissolved  out  more  rapidly  than  the  unexposed,  but  that  this 
relation  was  reversed  on  continuance  of  the  ammonia  "devel- 
opment." Luppo-Cramer  modified  Englisch's  experimental 
conditions  by  using  ammonia  vapor  instead  of  aqueous 
solutions.  This  has  advantages  in  that  the  action  is  slower 
and  therefore  can  be  better  observed,  and  that  there  can  be  no 
actual  removal  of  dissolved  silver  bromide  from  the  plate. 
Proceeding  in  this  way,  and  with  the  help  of  the  microscope, 
Luppo-Cramer  concluded  that  ammonia  development  is 
really  a  reaggregation  or  "ripening"  process  which  proceeds  at 
different  rates,  according  to  the  exposure  to  light  of  a  given 
part  of  the  plate. 

Luppo-Cramer  considers  that  this  supports  the  theory  that 
light  brings  about  a  certain  disintegration  of  the  silver  halide. 
He  ascribed  the  "developability"  with  ammonia  to  the  in- 
creased "inner  dispersity"  of  the  silver  bromide  grains.  He 
finds  that  at  first  the  exposed  parts  show  a  coarsened  grain, 
and  concludes  that,  in  consequence  of  disintegration,  the 
exposed  silver  bromide  particles  have  on  the  whole  a  greater 
solubility  in  ammonia,  whereby  at  first  an  immediate  Ostwald 
ripening  occurs.  This,  however,  is  reversed  on  further 
treatment,  the  unexposed  parts  becoming  later  relatively 
coarser-grained  than  the  exposed  parts.  This  he  attributes 
to  the  "disintegration  by  light"  affording  a  greater  number 
of  crystallization  nuclei,  whence,  with  greater  number  of 
crystals  formed,  the  ultimate  size  will  be  smaller,  since  the 
mass  of  material  per  unit  area  is  the  same.  Liippo-Cramer 
later  supported  this  view  with  experiments  in  which  the 
"chemical  latent  image"  was  completely  (?)  destroyed,  but 
could  still  be  developed  with  ammonia. 

It  does  not  appear  that  this  reasoning  is  either  necessary 
or  sufficient.  To  begin  with,  if  the  first  effect  in  the  more 
exposed  parts  is  essentially  an  increased  solubility  and  solution 
of  the  disintegrated  particles  of  the  original  silver  halide 
grains,  whence  come  the  subsequently  invoked  greater  number 

1  Englisch,  E.,  Zeits.  wiss.  Phot.  2:  416.  1905. 

2  Luppo-Cramer,  Photographische  Probleme,  p.  83. 

12 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

of  crystallization  nuclei?  And  also,  where,  at  the  same  time, 
are  the  relatively  larger  .  crystals  which,  according  to  the 
Ostwald  ripening  theory,  must  be  present  to  increase  at  the 
expense  of  the  smaller  crystals?  There  appear  to  be  mutually 
incompatible  requirements  here,  since  the  increased  crystal 
fragments  must  disappear — by  solution — to  give  the  postu- 
lated initial  Ostwald  ripening  in  the  more  exposed  parts,  and 
yet,  to  explain  the  apparent  reversal  effect  later,  must  also 
have  been  there  all  the  time. 

The  experiments  now  to  be  described  show,  we  believe, 
that  the  facts  are  capable  of  a  less  involved  explanation. 
They  show: 

(a)  That  there  is  no  ammonia  development  of  the  latent 
image,  properly  so  called,  but  only  an  ammonia  development 
of  the  visible  image,  no  effect  being  obtainable  with  exposures 
to  light  much  lower  than  those  which  give  the  least  visible 
photochemical  effect ; 

(b)  That   the   actual   development   can   be   more   simply 
explained  by  a  simple  recrystallization  effect,  not  involving 
directly,  but  only  (if  at  all)  as  a  very  subsidiary  factor,  any 
Ostwald  ripening; 

(c)  That  the  development  or  ripening  nuclei  are  due  not 
to   disintegration,   but   to   the   photochemical   decomposition 
products  of  the  silver  halide — probably  colloid  silver  adsorbed 
to  silver  halide — and  to  similar  decomposition  products  from 
the  reducing  action  of  the  ammoniacal  gelatine  on  the  silver 
halide. 

In  addition  to  correcting  what  appears  to  us  the  incorrect 
and  unnecessary  conclusions  drawn  by  Luppo-Cramer  in  his 
otherwise  valuable  and  interesting  papers,  the  experiments 
are  noteworthy  because  this  ripening  with  ammonia  affords 
a  cross-section  of  the  ripening  process  in  general,  particularly 
as  convection  currents  within  the  emulsion  are  eliminated. 
At  the  same  time,  it  is  believed  that  they  indicate  the  causes 
for  some  of  the  natural  limitations  and  peculiarities  in  the 
ripening  process. 

FUMING    OF   UNEXPOSED   LAYERS 

The  experimental  method  followed  was  in  the  main  similar 
to  that  of  Eder  and  Luppo-Cramer,  namely,  fuming  with 
ammonia  vapor  evolved  from  strongest  aqueous  ammonia. 
Some  side  experiments  with  ammonia  solutions  applied  direct 
showed  that  far  less  control  was  obtainable  in  this  way.  The 
aqueous  ammonia — S.  G.  0.90-0.92 — was  contained  in  deep 
crystallization  dishes,  the  plates  to  be  fumed  being  laid  film 

13 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

down  over  them,  so  that  an  unfumed  surround  was  obtained 
in  each  case.  Care  was  taken  that  the  distance  between 
the  ammonia  solution  and  the  plate  was  kept  constant,  unless 
purposely  varied.  For  convenience  the  fuming  was  conducted 
under  a  hood  in  a  dark  room.  Illumination  and  inspection 
during  the  experiments  were  facilitated  by  placing  the  care- 
fully leveled  dish  over  a  dark-room  lamp  laid  on  its  back,  so 
that  red  light  was  thrown  upward  through  the  dish  and  plate. 
In  all,  some  hundred  experiments  were  performed,  using  Seed 
23  and  Seed  Process  Plates,  Cine  Positive  film,  and  Lantern 
Plates. 

In  repeating  some  of  the  experiments  previously  described, 
an  initial  phase  of  the  action  of  ammonia  on  the  silver  halide 
emulsion  was  noted,  which  appears  to  have  attracted  little, 
if  any,  attention.  On  fuming. Seed  Process  plates,  unexposed 
to  light  and  unmoistened,  in  the  manner  described  above,  it 
was  observed  that  the  first  visible  differentiation  of  the  fumed 
from  the  unfumed  area  is  a  uniformly  diminished  opacity; 
this  was  such  that  in  one  hour — the  actual  time  varies  both 
with  the  kind  of  plate  and  with  its  relative  moisture  content— 
the  film  had  become  almost  transparent  by  transmitted  light, 
but  showed  a  light  bluish  gray  turbidity  by  reflected  light. 


FIG.  1  FIG.  2 

Print  through  fumed  plate  on  Print  on  fumed  plate  through 

unfumed  plate.     Crescent  shows  unfumed  plate.     Crescent  shows 

untreated  portion.  untreated  portion. 

The  extent  of  this  induced  transparency  is  shown  by  the 
photograph  in  Fig.  1,  which  gives  the  result  of  printing  a 
negative  through  a  plate  so  treated  on  to  another  one  unfumed. 

14 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

The  part  made  transparent  becomes,  photographically,1 
greatly  reduced  in  speed  and  density-giving  power,  as  is  shown 
by  Fig.  2,  which  gives  a  direct  print  from  the  negative  shown 
in  Fig.  1  on  the  partially  fumed  plate.  The  unfumed  sur- 
round is  overexposed  long  before  the  fumed  part  gives  a  devel- 
opable impression.  On  development  the  image  frequently 
shows  considerable  irregularly  distributed  surface  fog  of  a 
dichroic  nature.  Microscopic  investigation  of  the  fumed 
transparent  area  showed  that  in  this  state  the  emulsion  has  a 
fine  and  very  uniform  grain,  apparently  considerably  finer 
than  the  original.  The  reduction  of  opacity,  however,  is 


•  •«* 

IbgJfedfe        mm- 

^  «0P  ..  *B^ 


FIG.  3 
Ammonia  fuming,  early  stage 

due  not  solely  to  this  diminution  in  grain  size,  but  largely  to 
an  approximation  of  the  refractive  index  of  the  grains  to  that 
of  the  gelatine. 

The  photographic  speed-  and  density-giving  power  of  an  emulsion  on  exposure  and 
development  must  not  be  identified  with  the  photochemical  sensitiveness  giving  rise  to  a 
visible  image. 

15 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 


It  is  very  probable  that  at  this  stage  a  double  compound 
of  silver  halide  and  ammonia  is  formed.  (See  p.  25.)  The 
change  of  grain  size  is,  however,  not  very  pronounced  as 
compared  with  later  stages.  This  is  evident  in  that  at  this 
stage  the  change  is  largely  reversible.  On  being  removed 
from  the  ammonia  atmosphere  the  emulsion  regains  opacity 
to  near  its  former  value,  this  being  accelerated  by  an  air 
current.  The  next  phase  (on  continued  fuming),  is  an  irre- 
versible ripening,  in  the  sense  of  increase  in  size  of  grain, \m 
which  large  crystal  aggregates  are  formed.  They  commence 
at  isolated  points  (see  Fig.  3),  and  radiate  from  these  until  the 
respective  recrystallization  circles  or  domains  meet,  when 


FIG.  4 
Ammonia  fuming,  middle  stage 

boundaries  which  tend  to  be  straight  lines  are  formed,  so  that 
the  original  recrystallization  areas  become  polyhedral,  as 
illustrated  in  Figs.  4  and  5. 

It  will  be  seen  that  the  final  stage  is  a  complete  filling  up  of 
the  area  fumed  with  a  number  of  polyhedral  cells  enclosing  a 

16 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 


sort  of  efflorescence  of  trichites  and  crystal  aggregates.  The 
figures  so  far  given  are  from  natural  size  contact  prints  from 
contact  negatives  made  direct  from  the  original  preparations; 
hence  the  appearance  of  the  original  preparations  is  reproduced 
as  accurately  as  possible.  Photomicrographs  dealing  with  cer- 
tain aspects  of  this  recrystallization  process  will  be  given  later 
in  connection  with  the  discussion  of  the  theory.1  At  this 
point  it  is  necessary  only  to  note  that  the  beginning  of  nuclea- 


FIG.  5 
Ammonia  fuming,  final  stage 

tion,  under  the  conditions  given,  is  to  a  certain  extent  acci- 
dental. In  any  case,  it  commences  at  the  boundary  where 
the  film  is  in  contact  with  the  ammonia  container,  but  in  the 
fumed  area  dust  particles  or  other  casual  nuclei  seem  to  serve. 
As  scratching  the  sides  of  the  container  starts  crystallization 
from  solutions,  so  a  stress  mark  made  with  a  glass  rod  on  the 
emulsion  induces  ammonia  development  along  the  trace.2 

1  Examples  of  this  have  been  given  by  Eder  (I.e.)  and  Luppo-Cramer,  Kolloidchemie 
und  Photographic.     XII.     Koll.-Zeits.  9:  240.  1911. 

2  Cf.  Luppo-Cramer,  Kolloidchemie  und  Photographic,  I.e.     This  was  confirmed  in 
the  present  investigation. 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 
FUMING   EXPOSED   PLATES 

The  plates  were  exposed  behind  a  scale  negative — a  sensi- 
tometer  strip — to  a  100-watt  lamp  for  definite  times  and  at 
definite  distances.  From  density  measurements  of  the  scale 
negative  the  relative  exposures  could  be  calculated,  but  it  is 
unnecessary  to  give  these  within  this  scale,  because  it  was 
found  that  precision  in  the  matter  of  gradation  within  the 
scale  was  neither  important  nor  practicable  to  determine. 
As  has  been  pointed  out  under  conditions  of  fuming,  there 
is  no  actual  increase  or  decrease  of  material  within  a  given 
area,  exposed  or  unexposed,  but  only  changes  of  aggregation 
or  dispersity  and  of  refractive  index,  which  produce  an  appar- 
ent change  of  density  or  opacity.  Plates  were  exposed  both 
moist  and  dry.  In  preparing  the  moist  plates  the  plate  was 
soaked  for  a  brief  period  in  water,  and  superfluous  water 
blotted  off.  The  plates  were  weighed  dry  and  wet  to  deter- 
mine the  amount  of  water  absorbed. 

The  general  effect  of  moisture  was  greatly  to  accelerate 
the  action  of  ammonia  vapor.  Although  a  definite  propor- 
tionality of  effect  could  not  be  ascertained,  it  was  evident  that 
excessive  swelling  in  water  produced  more  irregular  effects. 
The  most  marked  difference  between  dry  and  moist  plates,  in 
line  with  the  acceleration,  was  the  much  coarser  grain  produced 
in  the  wet  or  moistened  plates,  as  will  be  evident  from  figures 
to  be  given  later. 

EXPOSURE    NECESSARY   FOR   AMMONIA   DEVELOPMENT 

A  result  of  importance,  in  view  of  earlier  pronouncements 
on  the  ' 'development  of  the  latent  image  by  ammonia,"  was 
that  the  exposures  to  light  necessary  to  obtain  a  developed 
image  were  of  an  entirely  different  order,  being  very  many 
times  greater  than  those  required  to  obtain  a  developable 
image  by  ordinary  development. 

Thus  with  Seed  Process  plates  the  exposure  necessary  to 
obtain  ammonia  development  of  an  image  of  the  scale  was 
some  150  times  that  necessary  to  give  an  image  developable 
with  pyro-soda.  With  Cine  Positive  film,  the  corresponding 
figure  was  about  250  times  as  long  an  exposure;  and  similar 
results  were  obtained  with  other  emulsions.  (With  lantern 
plate,  190  times.)  Under  these  conditions,  which  imply 
exposures  well  toward  the  ordinary  solarization  limit,  it 
appears  incorrect  to  speak  of  a  development  of  the  latent 
image  by  ammonia.  And,  in  fact,  close  inspection  showed 
that  ammonia  development  of  an  image  is  possible  only  from 
an  exposure  which  is  either  the  same  or  but  little  below  the 

18 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

threshold  value  of  exposure  to  give  a  visible  image.  Of 
course  this  threshold  varies  very  considerably  with  the  visi- 
bility conditions,  and  it  is  generally  possible  to  detect  with 
the  microscope  definite  evidence  of  photochemical  changes 
well  below  the  visible  threshold  of  image  formation. 

Fig.  6  shows  the  slight  indication  of  image  formation 
after  ammonia-fuming  a  Seed  Process  plate,  dry,  for  some 
seventeen  hours,  the  plate  having  received  an  exposure  130 
times  that  necessary  to  give  a  full  scale  with  pyro-soda  devel- 
opment. The  ammonia  development  here  has  proceeded  to 


FIG.  6 
Exposed  plate,  fumed  dry;  exposures  in  candle-meter-seconds 

the  stage  of  reversal  already  referred  to,  but  has  brought  out 
nothing  further  on  the  scale.  An  unfumed  control  plate  as 
well  as  the  plate  used  showed  a  threshold  visible  image  within 
ajfield  or  two  of  the  lowest  developed  by  ammonia. 

EFFECT   OF   MOISTURE 

If  the  plate  is  moistened  by  swelling  in  water,  fuming  with 
ammonia    produces   an    effect   earlier,    but    the   developable 

19 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

threshold  exposure  is  much  higher.  Thus  the  result  shown  in 
Fig.  7  was  obtained  after  soaking  a  plate  for  one  minute  in 
water  and  fuming  fifteen  minutes;  but  the  ill-defined  differenti- 
ation or  development  covers  only  part  of  the  scale  covered  in 
Fig.  6  and  implies  about  nine  times  as  great  an  exposure,  the 
normal  image  being  visible  over  a  greater  range.  Further 
development  with  ammonia  in  this  case  only  filled  the  plate 
with  crystal  aggregates  and  obliterated  the  primary  differen- 
tiation between  exposed  and  unexposed  portions.  The  faint 
indication  of  an  image  obtained  in  this  way  is  shown  in  Fig.  7. 


FIG.  7 

Exposed  plate,  fumed  moist;  exposures  in 
candle-meter-seconds 


REVERSAL 

The  appearance  of  reversal  observed  by  Luppo-Cramer  is 
clearly  indicated  in  Fig.  8.  It  is  to  be  noticed  that  more  than 
one  type  of  reversal,  as  regards  relative  optical  density,  is 
apparent  in  the  process.  The  exposed  portions,  as  compared 
with  the  unexposed,  appear  at  first  more  transparent,  and 

20 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

later,  less  transparent,  than  contiguous  unexposed  portions. 
This  is  due,  first,  to  the  new  silver  bromide-ammonia  complex 
having  a  lower  refractive  index  than  silver  bromide;  second, 
to  the  varying  stages  of  dispersity  of  the  new  and  old  phases. 
Reversal  with  increased  exposure  to  light  for  the  same  time 
of  development  (fuming)  indicates  that  the  optical  opacity 
at  first  increases  with  the  number  of  nuclei,  reaches  a  maxi- 
mum, and  then  diminishes.  (See  Fig.  8.)  Reversal  with 
increase  of  time  of  fuming  is  more  apparent  than  real,  being 


FIG.  8 
Exposed  plate,  showing  appearance  of  reversal 


dependent  upon  the  relations  between  the  stages  of  recrystal- 
lization  in  two  contiguous  fields.  Finally,  this  is  affected  by  a 
third  factor — partial  or  complete  reconversion  of  the  silver 
bromide-ammonia  complex  into  silver  bromide,  leaving 
pseudomorphs  of  silver  bromide  by  evaporation  of  ammonia. 

21 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 


EFFECT   OF   AMMONIA-FUMING    ON   DIFFERENT   EMULSIONS 

As  already  noted,  the  rate  and  sensibility  of  ammonia- 
fuming  is  very  dependent  in  one  and  the  same  emulsion  on 
the  actual  state  of  the  plate  in  respect  of  moisture  content. 
It  is  very  difficult  to  bring  different  emulsions  to  the  same 
state  in  this  respect,  hence  reliable  comparisons  between 
different  emulsions  are  anything  but  easy  to  obtain.  It  was 
hoped  at  one  time  that  ammonia-fuming  might  be  used  as  a 
method  of  investigation  and  control  of  the  "grain"  of  an 
emulsion  when  coated,  somewhat  in  the  manner  of  the  etching 
reactions  in  metallurgy;  but  it  is  evident  that,  even  if  it  should 
be  possible,  much  more  work  on  the  control  of  conditions  will 
be  necessary.  Taken  by  and  large,  however,  the  results 
showed  that  the  finer-grained  emulsions  react,  or  rather 
reaggregate,  more  rapidly  on  fuming  with  ammonia  than  the 
coarser-grained  ones.  Their  sensibility  in  the  matter  of  the 
development  of  an  image  by  ammonia  after  exposure  to  light 
appears  to  be  entirely  a  matter  of  their  photochemical  sensi- 
tiveness. The  rate  at  which  ammonia-ripening  takes  place 
is  a  function  of  the  size  of  the  grain,  the  character  of  the 
emulsion,  and  the  moisture  content. 

THEORY   OF   AMMONIA   DEVELOPMENT 

Reference  has  already  been  made  and  certain  objections 
raised  to  Luppo-Cramer's  theory  that  ammonia  development 
is  due  to  a  disintegration  of  the  silver  halide  grains  by  light. 
The  fact  that  in  the  absence  of  light  action  the  reaggregation 
by  fuming  starts  at  the  point  of  contact  of  the  vessel  used 
with  the  emulsion  layer,  or  within  this  at  casual  dust  particles 
or  other  nuclei,  suggests  that  it  is  unnecessary  to  postulate 
either  disintegration  of  silver  halide  grains  by  light,  or  Ostwald 
ripening.  The  simplest  explanation  is  that  reaggregation 
and  recrystallization  are  initiated  by  nuclei  furnished  by 
light.  Since  the  development  practically  only  commences 
with  exposures  giving  the  threshold  of  a  visible  image,  it  is 
evidently  unnecessary  to  look  for  these  nuclei  further  than  the 
photochemical  decomposition  product,  most  probably  colloid 
silver  adsorbed  to  residual  silver  halide,  forming  photo-halide. 
Accepting  this,  and  in  view  of  the  absence  of  ammonia  devel- 
opment for  exposures  much,  if  at  all,  below  the  visibility 
threshold  of  light  action,  it  appears  that  the  nuclei  in  the 
range  of  the  so-called  latent  image  are  either  not  large  enough 
or  possibly  still  too  "protected"  by  residual  silver  halide  to 
function  in  ammoniacal  recrystallization.  This,  however, 
is  only  in  line  with  the  fact  that  the  threshold  values  of 

22 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

exposures  above  which  it  is  possible  to  develop  physically  by 
an  acid  silver  developer  is  very  much  higher  (particularly 
in  coarser-grained  plates),  than  that  for  ordinary  chemical 
development. 

The  evident  reversal  on  prolonging  development  is  simply 
a  consequence  of  the  variation  of  light  transparency  with  the 
phase  of  the  recrystallization  process  and  the  dispersity  of  the 
reaggregated  silver  halide.  In  the  first  noticed  phase  of 
apparent  homogeneous  peptization  and  increased  transpar- 
ency, new  crystal  nuclei  of  an  ammonia-silver  halide  complex 
start  to  form  about  nuclei  furnished  by  light.  Then  the  size 
of  these  nuclei  increases,  and  at  first  therewith  the  opacity. 
But  in  less  exposed  regions,  and  a  fortiori  in  non-exposed  ones, 
the  number  of  initial  foreign  nuclei  is,  at  the  start,  propor- 
tionately less;  hence  the  grain  size  on  reaggregation  can 
overtake  that  in  the  exposed  regions  where  there  are  a  greater 
number  of  nuclei.1  But  a  limit  is  set  to  this  and  a  tendency 
to  neutralize  the  initial  differentiation  formed  by  the  fact 
that  prolonged  action  of  ammonia  on  the  gelatino-silver 
halide  results  in  a  chemical  reduction,  thus  furnishing  colloid 
silver  nuclei  which  are  sometimes  evident  as  a  silver  stain 
after  fixing,  but  which  are  generally  developable  with  a  silver 
intensifier  after  careful  fixation  and  washing.  Luppo-Cramer's 
chief  argument  for  the  disintegration  hypothesis  is  the  possi- 
bility of  ammonia  development  after  destruction  of  the  image, 
as  evidenced  by  incapacity  for  development  with  free  silver, 
the  destruction  being  brought  about  by  bromine.  In  repeating 
these  experiments  it  was  found,  first,  that  the  threshold 
exposure  which  could  be  differentiated  by  ammonia  was 
much  raised  by  this  treatment,  and,  secondly  (as  stated  by 
Luppo-Cramer),  that  the  differentiation,  or  development, 
is  very  imperfect  after  this  treatment.  This  result  is  in  no 
way  a  necessary  consequence  of  the  disintegration  theory. 
It  is  equally  well  and  perhaps  better  accounted  for  on  the 
colloidal  silver  nuclei  theory  here  proposed.  While  bromina- 
tion  tends  to  re-halogenize  the  photochemical  decomposition 
product,  the  silver  halide  thus  formed  is  not  physically  homo- 
geneous with  the  original  silver  halide  grains,  but,  as  altered 
material,  may  itself  furnish  nuclei  for  the  ammonia  recrystal- 
lization. Further,  and  perhaps  more  effective,  is  the  local 
reaction  on  the  gelatine. 

1  If  the  plate  is  fumed  moist,  then  dried  out  again,  the  visual  opacity  of  the  exposed 
regions  is  usually  higher  than  that  of  the  unexposed  region,  and  increases  to  a  limit  with 
exposure.  If  the  plate  is  fumed  dry,  and  further  dried  out  after  fuming,  the  opacity  of  the 
lower  exposures  is  usually  less  than  in  the  unexposed  region  adjacent,  reaches  a  minimum, 
and  then  increases  again,  but  usually  does  not  reach  that  of  the  adjacent  unexposed  region. 
The  phenomena  are  further  varied  by  the  nature  of  the  emulsion  and  the  original  size  of  the 
emulsion  grains. 

23 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

The  conception  that  the  new  phase  formed  by  photo- 
chemical decomposition  could  furnish  the  nuclei  for  this 
recrystallization  was  tested  indirectly  in  two  ways.  First, 
a  plate  was  given  an  exposure  just  sufficient  to  form  a  latent 
image,  i.  e.,  one  developable  with  a  chemical  developer,  but 
not,  as  already  stated,  with  ammonia.  This  image  was  just 
developed,  very  faintly,  but  not  fixed.  After  washing  and 
drying,  the  plate  was  fumed  with  ammonia,  whereupon  a 
well-defined  image  was  developed  up,  showing  that  the  silver 
nuclei  furnished  by  development  could  function  as  nuclei  for 
ammoniacal  recrystallization.  See  Fig.  9. 


FIG.  9 

Normally  exposed  plate,  developed  to  appearance  of  image  in 
diluted  developer,  then  fumed  with  ammonia 

As  a  second  indirect  support  of  the  theory  advanced,  the 
development  of  colloidal  gold  nuclei  by  ammonia-fuming  can 
be  brought  forward.  By  marking  a  plate  with  a  glass  rod 
dipped  in  gold  chloride  solution  and  drying  down,  then  washing 
well  and  drying  again,  a  very  faint  deposit  of  colloidal  gold  is 

24 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

left.     On  fuming  with  ammonia,  this  is  developed  by  recrystal- 
lization  on  the  traces  of  gold. 

The  minutiae  of  the  recrystallization  and  reaggregation 
process  induced  by  ammonia-fuming  will  be  discussed  later 
in  connection  with  the  general  theory  of  ripening.  Apart 
from  the  general  interest  of  the  phenomena  in  question,  the 
phase  of  increased  transparency  first  noted  may  be  worth 
investigating  sensitometrically.  If  a  homogeneous  pepti- 
zation  is  effected,  the  resolving  power  and  solarization  limit 
should  be  markedly  altered.  Another  point  of  interest  is 
the  relation  of  this  ammonia  recrystallization  to  the  ripening 
of  emulsions.  The  reaggregation  or  alteration  of  dispersity 
will  be  discussed  later,  but  it  should  be  mentioned  here  that 
the  existence  of  direct  chemical  reduction  of  the  silver  halide 
by  ammonia  and  gelatine  combined  was  found.  It  is  probable 
that  this  plays  a  determining  role  in  the  occurrence  of  both 
ripening  fog  and  aging  fog  in  gelatino-bromide  emulsions. 

The  chemical  composition  and  constitution  of  the  silver 
halide-ammonia  compounds  is  quite  fully  discussed  by 
Ephraim1  in  his  studies  on  auxiliary  valences.  He  concludes 
that  a  maximum  of  three  ammonia  molecules  can  become 
attached  to  the  silver  atom,  so  that  for  salts  saturated  with 
ammonia  the  composition  will  be  AgHal:  3NH3,  while  the 
constitution  may  be  any  one  of  the  following: 

I.  [Ag(NH,),X];          II.  [Ag(NH3)6AgX2];         or  III.   [AgC^]. 

At  ordinary  temperatures  the  tri-amine  is  not  stable, 
passing  over  to  the  di-  and  mono-amines  as  the  temperature 
is  increased  or  as  the  pressure  of  ammonia  is  diminished. 

SUMMARY 

1.  The   general   course   of   development   of   silver   halide 
emulsions  by  ammonia  was  found  to  be  similar  to  that  de- 
scribed by  Eder  and  Luppo-Cramer. 

2.  It  appears  to  be  incorrect  to  speak  of  "development  of 
the  latent  image"  in  this  connection,  as  the  ammonia  devel- 
opment does  not  begin  much,  if  at  all,  below  the  threshold  of 
the  visible  (print-out)  image. 

3.  It  is  concluded  that  the  process  consists  primarily  in 
recrystallization  of  the  silver  halide  as  a  silver  halide-ammonia 
complex  on   nuclei   furnished   by   the  visible  photochemical 
image;  it  is  therefore  unnecessary  to  assume  either  a  mechan- 

1  Ephraim,  F.,  Ueber  die  Natur  der  Nebenvalenzen.     XIX.     Ammoniakate  des  Silbers. 
Ber.  chem.  Gesell.  51:  706.  1918. 

25 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

ical  disintegration  of  the  grains  by  light  or  Ostwald  ripening 
as  factors  in  the  effect. 

4.  The  following  results  appear  of  importance  for  the 
general  theory  of  emulsions :  The  opacity  to  light  of  a  mass 
of  silver  halide  increases  at  first  on  recrystallization  with  the 
number  of  independent  nuclei.  Independent  nuclei  can  be 
furnished  by  foreign  substances,  such  as  colloidal  silver  or 
gold,  or  probably  even  altered  gelatine.  Under  the  combined 
action  of  ammonia  and  gelatine  silver  bromide  is  reduced 
with  production  of  colloidal  silver. 


26 


CHAPTER  II 

Von  Weimarn's  Theory  and  the  Determination  of 
the  Dispersity  of  Silver  Bromide  Precipitates 

The   sensitive   silver   halide   preparations  used  in  photo- 
graphy may  be  divided  into  two  main  classes: 

A.  Silver  halide  formed  in  the  presence  of  excess  silver  salt. 
This  includes  wet  collodion,   collodion   emulsion,   and   most 
printing-out   emulsions.     The   function   of  excess   silver   salt 
here  is  probably  chiefly  that  of  a  chemical  sensitizer,  i.  e.,  as  a 
halogen  absorbent; 

B.  Silver  halide  formed  in  the  presence  of  excess  soluble 
halide.     This   includes  both   positive   and   negative   gelatine 
emulsions    for    chemical    development.     While    development 
emulsions  for  printing  (developing-out  papers)  and  positives 
depend    chiefly    upon    silver    chloride    and    combinations    of 
silver  chloride  and  silver  bromide  in  which  the  after-process 
of  ripening  plays  a  relatively  small  part,  the  fundamentally 
important  negative  emulsions  are  composed  of  silver  bromide 
and  silver  iodide,  the  silver  bromide  in  considerable  excess  and 
seldom  used  alone.     By  ripening  is  understood  the  increase 
in  speed  and  change  in  other  sensitometric  properties  induced 
by  certain  digestion  processes,  either  by  heat,  with  excess  of 
soluble  bromide  present  (boiling  process),  or  at  lower  tempera- 
tures  by   ammonia.     This   treatment  generally   involves   an 
increase  in  the  average  size  of  the  grains,  or,  in  the  terms  of 
colloid  chemistry,  a  decrease  in  the  dispersity.     It  was  at  one 
time  associated  with  the  flocculation  of  colloid  particles,1  but 
later  has  been  more  generally  regarded  as  a  case  of  Ostwald 
ripening.     By  this  is  meant  the  growth  of  larger  crystalline 
particles  at  the  expense  of  smaller  ones,  on  the  presumption 
that  the  latter  have  a  greater  solubility.     Before  considering 
either  the  general  grounds  for  this  thesis  or  its  specific  applica- 
bility to  photographic  emulsions,   it  should  be  pointed  out 
that  modern  high-speed  emulsions,  relatively  coarse-grained, 
are  not  produced  by  the  ripening  of  emulsions  which  would 
otherwise  be  slow  and  fine-grained.     The  two  types  are  pro- 
duced under  relatively  different    initial  conditions,  and,  as 
pointed   out  by  Luppo-Cramer2  and   Mees,3   are  practically 
discontinuous. 

1  Cf.  Quincke,  G.,  Die  Bedeutung  der  Oberflachenspannung  fur  die   Photographic  mit 
Bromsilbergelatine  und  eine  Theorie  des  Reifungsprozesses  der  Bromsilbergelatine.     Jahrb. 
Phot.   19:  3.   1905. 

2  Luppo-Cramer,  Photographische  Probleme,  I.e. 

3  Mees  C.  E.  K.,  The  physics  of  the  photographic  process.     J.  Frankl.  Inst.  179:   141. 
1915. 

27 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

It  may  be  said  that  fundamentally  the  production  of  such 
different  emulsions  depends  upon  certain  general  principles 
for  regulating  the  dispersity,  or  average  grain  size,  of  a  solid 
precipitate.  In  a  certain  measure  specific  applications  of 
these  principles  have  been  familiar  for  a  long  time  to  both 
analytical  and  industrial  chemists.  But  it  is  only  in  recent 
years,  with  the  development  of  colloid  chemistry,  that  they 
have  been  reduced  to  definite  and  general  laws,  capable  to  a 
certain  extent  of  mathematical  expression,  and  equally  con- 
cerned with  the  genesis  of  colloids  and  of  crystals.  While 
the  conditions  for  crystallization  from  liquid  melts  by  cooling 
have  become  known  largely  through  the  researches  of 
Tammann,1  the  determination  of  similar  principles  governing 
the  dispersity  or  internal  subdivision  of  a  new  phase  separating 
from  supersaturated  solutions,  particularly  where  the  new 
phase  is  solid,  is  due  to  the  Russian  investigator,  von  Weimarn/2 
His  contentions  as  to  the  "vectorial"  character  of  phases 
of  matter  usually  termed  amorphous  will  be  noted  later. 
Meanwhile  the  kernel  of  his  work  will  be  briefly  reviewed,  as 
it  is  of  considerable  experimental  and  practical  importance. 

It  is  a  fact  well  known  to  chemists  that  not  only  does  the 
form  and  subdivision  of  a  precipitate  vary  in  different  sub- 
stances, but  that  variations  in  the  conditions  of  precipitation 
will  alter  the  character  of  the  precipitate  for  one  and  the  same 
substance.  Thus,  Stas3  distinguishes  certain  "modifications" 
of  the  silver  halides,  reference  to  which  is  made  in  many 
text-books  of  photography.4 

Von  Weimarn's  first  postulate  is  that  the  actual  form  and 
internal  subdivision  of  a  new  solid  phase  are  determined  by 
two  sets  of  factors : 

1.  Unilateral  influence  of  the  vectorial  molecular  forces 
on  the  molecules  forming  the  free  (crystal)  surface.  By  this 
is  meant  a  directing  or  orienting  force  of  the  molecules  separ- 
ating as  a  new  phase  on  those  forming  the  free  surface  of  the 
crystalline  individuals  of  this  phase.  It  is  considered  that 
the  molecules  in  the  free  surface  of  a  crystal  are  imperfectly 
oriented,  or  imperfectly  ordered  in  respect  of  the  space  lattice 
determining  the  crystal  system  and  form.  Since  the  smaller 
the  crystal,  the  greater  its  surface  as  compared  with  its  volume, 
the  crystalline  ordering  would  tend  to  be  overwhelmingly 
deviated  from  if  it  were  not  for  this  factor.  It  is  considered 

1  Tammann,  G.,  Krystallisieren  und  Schmelzen. 

2  Weimarn,  P.  von,  Zur  Lehre  von  den  Zustanden  der  Materie. 

3  Stas,  J.  S.,  Recherches  de  statique  chimique  au  sujet  du  chlorure  et  du  bromure 
d'argent.     II.    Ann.  chim.  phys.  V.  3:  145.   1874. 

4  Eder,  J.  M.,  Ausfuhrliches  Handbuch  der  Photographic,  Vol.  Ill,  p.  13. 

28 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

as  giving  rise  to  a  "capillary  pressure"  additive  to  the  general 
hydrostatic  pressure  on  the  surface,  and  increasing  with 
increasing  dispersity.  Hence,  in  general,  it  increases  the 
crystalline  solid  character,  raising  the  melting  point  as  the 
size  of  the  grain  diminishes  for  substances  for  which  pressure 
does  this,  while  for  the  relatively  few  substances,  like  water :  ice, 
the  opposite  obtains ; 

2.  The  second  factor  or  group  of  factors  is  considered  to 
be  always  tending  to  bring  the  substance  to  the  fluid  state — 
i.  e.,  one  of  relatively  unordered  molecular  movement — and 
is  sometimes  identified  with  ordinary  dynamic  surface  tension. 

The  conception  that  the  form  and  size  of  a  crystal  depend 
upon  a  balance  of  internal  and  external  forces  is  itself  logical, 
and  will  be  considered  from  a  slightly  different  angle  later. 
It  should  be  noted  at  this  point,  however,  that  von  Weimarn 
contends  that  all  so-called  amorphous  solid  precipitates  are 
essentially  crystalline  in  the  character  of  their  unit  particles, 
cellular  and  flocculent  textures  being  due  to  secondary  causes. 
The  crystallinity  of  the  particles  may  be  ultra-microscopic 
(krypto-crystalline) ,  but  it  exists  as  a  reality  determining 
the  trend  of  their  changes.1  Since  a  crystal  is  regarded 
essentially  as  a  phase  of  definite  composition,  this  standpoint 
is  in  apparent  contradiction  with  the  general  view  of  such 
precipitates  as  "absorption-compounds,"  i.  e.,  as  phases  of 
variable  composition.2  The  explanation  is  that  the  purity  or 
composition  of  a  crystal  is  largely  a  function  of  its  size.  The 
smaller  the  crystal,  the  larger  its  relative  surface,  and  the 
more  it  is  liable  to  contamination  with  dissolved  and  adsorbed 
foreign  molecules.  Hence,  the  composition  of  a  solid  crystal- 
line dispersed  phase  is  a  function  of  the  dispersity. 

The  composition  of  the  surface  layer  may  be  expressed  as 
Xn  Ym  Zp,  i.  e.,  Xn  molecules  of  the  dispersed  substance,  Ym 
molecules  of  the  solvent  (or  dispersing  medium),  Zp  molecules 
of  co-existing  solutes.  TV,  m  and  p  need  stand  in  no  rationally 
fixed  ratio;  thus  Ym  may  be  nearly  eliminated  by  drying, 
while  Zp  will  increase  with  the  dispersity  of  Xn  proper.  These 
combinations  form  the  class  of  adsorption  compounds  or 
"capillary  combinations." 

It  is  desirable  to  add  to  this  exposition  of  von  Weimarn's 
theory  that,  as  a  result  of  other  work,  we  regard  the 
"attachment"  of  the  components  Zp  and  Ym  as  varying  from 
a  state  of  true  solution  in  the  crystal  to  one  of  entirely  super- 

1  Cf.  von  Weimarn,  1.  c.,  vol.  I,  p.  13. 

2  Bemmelen,  J.  M.  van,  Die  Absorption. 

29 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

ficial  combination  by  chemical  (residual)  affinity.1  In  general, 
however,  the  contamination  will  be  ruled  by  the  following 
characteristics  : 

a.  Equilibrium  is  reached  very  rapidly; 

b.  The  reverse  separation  by  washing  with   pure  solvent   proceeds 
very  slowly,  adsorption  being  in  most  cases  practically  irreversible; 

c.  For  low  concentrations  of  the  adsorbed  substance  (in  the  solution) 
relatively  greater  amounts  are  adsorbed  than  at  high  concentrations. 

Von  Weimarn  considers  that  these  characteristics  are 
largely  explainable  by  the  fact  that  the  surface  layer  of  a 
crystal  behaves  in  a  measure  like  a  strongly  compressed 
viscous  liquid. 

Coming  now  to  a  more  specific  consideration  of  precipi- 
tation, he  points  out  that  the  actual  aggregation  of  the  mole- 
cules of  a  new  phase  depends  upon  a  considerable  number  of 
proximate  factors,  e.  g.,  its  solubility,  its  latent  heat  of  conden- 
sation, the  pressure  on  the  medium,  viscosity,  and  the 
concentration  of  reactants.  Of  these,  solubility  and  concen- 
tration demand  first  attention,  and  it  is  assumed  concerning 
them  that  aggregation  of  a  new  phase  may  be  divided  into 
two  stages,  the  first  (a)  consisting  in  the  formation  of  amicro- 
scopic  "germs"  or  nuclei,  the  second  (b)  in  the  growth  of 
these  particles,  chiefly  by  diffusion  of  dissolved  molecules 
into  the  sphere  of  their  attraction.  This  division  is  common 
to  Tammann's  theory  of  crystallization  from  super-cooled 
melts  and  von  Weimarn's  theory  of  crystallization  from 
supersaturated  solutions.  The  latter  proposes  the  parallel 
forms  : 

SUPERSATURATED   SOLUTIONS  SUPER-COOLED  LIQUIDS 

a.   (1st  stage) 

Condensation  pressure  _       Super-cooling 

Condensation  resistance  Latent  Heat 

~  T~T 


=  R      ~      =  P/S  =  E 

O  Li 

Where       W   —  velocity  of  condensation 

Q   =  total  available  molecules  in  solution 

S    =  normal  solubility  of  coarse-grained  phase 

Hence  Q  —  S   =  actual  supersaturation 

P/S   —  specific  supersaturation  at  initial  condensation 

b.   (2nd  stage) 

V  =   -  .  Z(C-c)  v  =  ?  .  Z(t-T),  where 

a  a 

Where         V   =  velocity  of  crystallization        v  =  velocity  of  crystal- 

lization 

1  Cf.  Langmuir,  I.,  The  constitution  and  fundamental  properties  of  solids  and  liquids. 
I.  Solids.     J.  Amer.  Chem.  Soc.  38:  2221.  1916;  II.  Liquids,  ibid.  39:  1848.  1917. 

30 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

D    =  diffusion  coefficient  H  =  heat  conductivity 

d    =  diffusion  path  length  d  =  diffusion  path  length 

c    =  saturation  concentration          t  =  melting  temperature 
(solubility)  for  a  given 
dispersity 
Z    =  total  surface 

The  expression  for  the  second  stage  is  essentially  identical 
with  that  for  the  so-called  Nernst  theory  of  heterogeneous 
reactions,1  which  may  be  considered  an  adaptation  of  Wilder- 
man's  formula.2 

Nernst's  modification  consisted  in  supposing  that,  in 
heterogeneous  chemical  reactions,  the  reaction  itself  was 
accomplished  with  practically  infinite  velocity  and  adjacent 
to  the  boundary  surface  between  phases,  and  that  the  velocity 
measured  was  that  of  diffusion  across  a  layer  d  having  a 
concentration  gradient  varying  from  saturation  to  that  general 
in  the  solution.3  Hence,  for  the  velocity  constant  k  he  sub- 
stituted D/d,  D  being  the  diffusion  coefficient. 

This  conception  has  been  keenly  criticized,4  and  in  fact 
is  only  partially  adequate  for  a  limited  number  of  cases.  Its 
insufficiency  will  be  specifically  noted  later.  As  regards  the 
second  stage  of  crystallization,  Wilderman's  generalized 
expression, 

V  =  k  Z  (C-c)  +  K, 

(where  C-c  expresses  the  concentration  difference,  or  distance 
from  equilibrium,  and  K  is  a  characteristic  "instability  con- 
stant"), may  be  substituted  without  affecting  von  Weimarn's 

thesis. 

The  following  important  deductions  are  made: 
I.  Provided  that  the  product  (volume  x  concentration)  be  kept  constant 
and  sufficient  time  allowed,  individual  crystal  magnitudes  are  inversely 
proportional  to  W,  the  initial  condensation  velocity; 

II.  With  increasing  W  the  number  of  nuclei  increases,  but  for  very  high 
W  adhesion  of  these  occurs — i.  e.,  groups  or  clumps  of  crystal  nuclei 
cohere,  forming  a  single  crystalline  aggregate  termed  by  von  Weimarn 
aggregation  crystallization. 

In  any  case,  the  initial  stage  of  separation  of  a  new  phase 
is  the  formation  of  a  colloid  solution  (suspensoid  or  emulsoid). 
This,  however,  may  be  so  transient  as  to  escape  notice,  depend- 
ing upon  the  relation  between  the  velocity  of  initial  conden- 
sation and  that  of  crystalline  growth. 

1  Nernst,  W.,  Theoretische  Chemie. 

2  Wilderman,  M.,  On  the  velocity  of  reaction  before  complete  equilibrium  and  before 
the  point  of  transmission,  etc.     Phil.  Mag.  VI.     2*:  50.     1901. 

3  Cf.  W.  Nernst,  1.  c. 

4  Cf.  M.  Wilderman,  1.  c. 

31 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

The  important  factor  here  is  not  the  absolute,  but  the 
specific  supersaturation  P/S.  Thus,  with  a  given  value  of  P 
(say  a  few  grains  per  100  cc.)  a  very  soluble  substance  (e.  g., 
sodium  chloride  in  water),  will  deposit  nothing  at  first,  even 
for  considerable  supersaturation,  since  not  only  is  the  solu- 
bility of  coarse-grained  crystalline  sodium  chloride  very 
considerable,  but  that  of  the  amicrons  is  even  greater.1  Hence 
the  initial  velocity  of  condensation  is  small  compared  with, 
for  instance,  silver  chloride. 

However,  the  value  of  P,  the  absolute  supersaturation, 
is  still  of  considerable  importance.  The  resulting  precipitate 
will  be  very  different,  according  as  a  given  value  of  P/S  (=  V) 
is  due  to  large  or  small  S.  In  the  one  case,  a  large  amount  of 
the  precipitate  is  formed,  in  the  other,  little.  If  V  be  large, 
the  former  case  gives  a  gelatinous  precipitate,  or  gel;  if  V  be 
small,  a  large  number  of  dispersed  particles,  therefore  a 
solution.  Thus  by  suitable  alteration  of  P  or  S,  or  both,  we 
can  ensure  the  initial  separation  of  the  dispersed  phase  in  any 
desired  form. 

The  stability  of  the  new  phase  in  the  initial  condition  is 
dependent,  to  a  large  extent,  upon  conditions  expressed  in 
the  formula  for  the  second  stage.2  The  smaller  the  existent 
supersaturation  C—c,  and  the  smaller  the  value  of  V,  the 
greater  the  stability.  Decrease  of  Z>,  the  diffusivity,  helps 
this.  Hence,  for  stable  suspensoid  hydrosols  there  are 
required : 

Large  values  of  P=Q—S/S  and 
Small  values  of  5 

so  that  V  may  be  large,  giving  many  nuclei. 

An  example  along  these  lines,  worked  out  in  detail  by 
von  Weimarn,  is  barium  sulphate.  The  solubility  in  water 
at  18°  C.  is  .00024  gms.  per  100  cc.  This  is  large  enough  so 
that,  with  ordinary  solutions  of  barium  salts  with  sulphates, 
the  values  of  P  do  not  give  large  values  of  V  and  hence  barium 
sulphate  is  obtained  in  an  immediate  microcrystalline  form. 
But  from  more  concentrated  solutions  of  more  soluble  barium 
salts,  e.  g.,  barium  sulphocyanide  and  manganese  sulphate, 
the  barium  sulphate  may  be  obtained  either  as  a  cellular  gel 
or  a  translucent  hydrosol. 

Summarizing  von  Weimarn's  postulates  at  this  stage: 
1.  With  very  soluble  substances,  suspensoids  are  obtained  only  for  large 

values  of    V,  resulting  in  a  gel.     If   V  be  small,  the  suspensoid  is 

transitory. 

1  The  presumption  of  a  higher  solubility  of  finer  grained  particles  will  be  discussed  later. 

2  It  must  be  understood  that  these  stages  are,  as  regards  the  general  rate  of  change  and 
the  total  mass  in  course  of  change,  continuous  and,  to  some  extent,  simultaneous.     The 
differentiation  is  mainly  important  as  affecting  the  character  of  the   precipitate. 

32' 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

2.  With  substances  of  small  but  measurable  solubility    (from  4  to  10 
gms.  in  100  cc.),  the  suspensoid  stage  is  reached  for  both  large  and 
small  values  of  V,  the  former  resulting  in  a  gel,  the  latter  in  a  dilute 
hydrosol. 

3.  With   substances   practically   insoluble   the  suspensoid   stage   is   not 
recognizable  for  small  values  of   V.     With  large  values  of   V,  dilute 
sols  are  obtained. 

THE    DETERMINATION   OF   THE   INITIAL   GRAIN  SIZE 
OF   PRECIPITATES   IN   RELATION   TO 

VON  WEIMARN'S  THEORY 

As  a  first  application  of  von  Weimarn's  principles,  the 
precipitation  of  silver  bromide  at  different  initial  concentra- 
tions was  studied  with  V  x  C  kept  nearly  constant.  Equiva- 
lent amounts  of  silver  nitrate  and  potassium  bromide  solutions 
were  mixed  at  a  temperature  of  20°  C.  A  deviation  was 
made,  simulating  emulsion  practice,  in  that  steady  mechanical 
agitation  was  employed.  The  vessels  were  dimensionally 
similar  cylinders  with  proportionate  Wulff  (centrifugal) 
agitators.  Under  these  conditions  it  was  not  found  that  the 
dispersity  of  the  precipitate  was  markedly  affected  whether 
the  silver  nitrate  was  added  to  the  potassium  bromide  or 
conversely.  However,  the  photochemical  sensitiveness  (visible 
darkening)  was  higher  in  the  latter  case.1 

The  results  obtained  are  shown  in  the  following  table: 

EQUIVALENT 
CONCENTRATION  DISPERSITY  REMARKS 

(Normal) 

.0002  Clear  hydrosol 

.  0004  Clear  hydrosol 

.0010  Clear  hydrosol 

.0025  Cloudy  sol,  settles  somewhat  Decreasing 

.025  Turbid  suspension,  settling  dispersity 

.  25  Flocculent   ->crumbly  ppt. 

.75  F,occu,ent 


1.50  Flocculent  ppt.  ->silt 

3.00  Curdy  ->  crumbly  ppt.  Decreasing 

4.50  Curdy,  quasi-cellular  voluminous  gel  dispersity 

See  outside  curves  in  Figs.  10,  11,  12. 

These  results  are  mainly  qualitative  and  will  be  studied 
quantitatively  later.  They  show,  however,  that  as  the  initial 
concentration  of  reactants  is  increased,  the  dispersity  passes 
through  a  minimum,  the  precipitate  here  obviously  approach- 
ing the  crystalline  condition.  And  for  lower  and  higher 
concentrations  it  tends  to  give  colloid  sols  and  gels  respectively. 

1  As  has  been  indicated  by  Mees  (The  Physics  of  the  Photographic  Process,  1.  c.), 
many  properties  of  the  emulsion  probably  depend  not  simply  on  the  dispersity  (grain-size), 
but  on  the  distribution  of  sizes,  or  "dispersity  gradient." 

33 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

Now  the  extent  to  which  the  variation  of  dispersity  thus 
offered  by  change  of  initial  concentration  is  photographically 
useful  is  limited.  For  one  thing,  it  is  not  practicable  to  keep 
the  volume  concentration  constant;  for  another,  the  practic- 
able range  of  amount  of  precipitate  per  unit  volume  of 
emulsion  is  limited.  Hence,  in  any  case,  other  factors  must 
be  introduced;  and  these  are  superposed  upon  variation  of 
initial  concentration  within  a  certain  limited  practicable 
range. 


34 


CHAPTER  III 

Accessory  Factors  Influencing  the  Dispersity 
of  Silver  Bromide  Emulsions 

In  von  Weimarn's  theory,  the  grain  size  of  precipitates  is 
considered  as  being  regulated  essentially  by  the  initial  concen- 
tration of  the  reactants.  In  point  of  fact,  there  are  a  number 
of  modifying  factors,  frequently  as  important  as  concentration, 
which  may,  for  convenience,  be  termed  accessory  dispersal 
factors.  These  factors,  in  so  far  as  silver  bromide  emulsions 
are  concerned,  are: 

1.  The  colloid  emulsifying  medium,  and  variation  of  its  concentration 

and  condition; 
2.-  Effect  of  mixture  of  silver  halides; 

3.  Addition  of  solubilizing  reagents,  in  particular  excess  of  bromide  or 
ammonia; 

4.  Addition  of  other  soluble  ingredients  acting  either  on  the  silver  halide 
or  on  the  gelatine  or  on  both,  and,  modifying  all  these,  temperature 
and  agitation. 

Taking  these  up  in  detail: 

1.    EFFECT    OF    COLLOID    MEDIUM    UPON    DISPERSITY 

The  influence  of  colloid  media  such  as  gelatine  upon  the 
silver  halide  precipitate  is  far-reaching.  To  begin  with,  silver 
halide  precipitated  in  the  absence  of  such  a  medium  is,  except 
where  certain  special  precautions  are  taken,  practically  imme- 
diately reducible  by  developers.  That  is,  it  is  not  only 
mechanically,  but  also  chemically,  unsuitable  for  photo- 
graphic purposes.  It  was  suggested  by  Sheppard  and  Mees1 
that  the  most  probable  explanation  of  this  form  of  the  pro- 
tective function  of  gelatine  is  that  it  acts  as  a  filter  against 
nuclei  (development  germs),  and  this  view  is  strongly  sup- 
ported by  Luppo-Cramer.2  Apart  from  this,  however,  it 
affects  the  dispersity  (or  size  of  grain)  and  the  form  and 
composition  of  the  individual  grain. 

Considering  dispersity  first,  we  have  at  present  only 
qualitative  indications.  It  is  well  known  that,  in  the  presence 
of  gelatine,  silver  bromide  is  still  obtained  as  a  colloid  hydrosol 
at  concentrations  of  the  reagents  which  would  otherwise  give 
a  coarse-grained  precipitate.  Generally,  it  appears  that  if 
the  relation  between  concentration  of  precipitants  and  dis- 

1  Sheppard,  S.  E.,  and  Mees,  C.  E.  K.,  I.e.,  p.  206. 

2  Luppo-Cramer,     Kolloidchemie     und     Photographic.     XIII.     Koll.-Zeits.  10:  182. 
1912.    Cf.  Reinders,  W.,  and  Nieuwenburg,  J.  van,  Gelatine  und  andere  Kolloide  alsVerzog- 
erer  bei  der  Reduktion  von  Chlorsilber.     Koll.-Zeits.  10:36.  1912. 

35 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

persity  of  new  phase  is  represented  by  a  curve  of  the  type 
shown  in  Figs.  10,  11  and  12,  i.  e.,  passing  through  a  minimum 
dispersity,  then  the  effect  of  a  protective  colloid  is  to  flatten 
out  the  curve  and  shift  the  minimum  more  or  less  considerably 
to  regions  of  higher  concentration.  This  effect  will  in  general 
be  the  more  pronounced  the  greater  the  concentration  of  the 


Concentrations  in  Normalities 

FIG.  10 

gelatine,  so  the  emulsion-maker  can  control  the  dispersity  to 
a  considerable  extent  by  varying  the  concentration  of  gelatine 
present  at  mixing,  supplying  the  rest  as  required. 

It  is  probable  that  part  at  least  of  this  effect  is  due  to  the 
increase  of  viscosity,  or  inner  friction,  of  the  medium.  Revert- 
ing to  von  Weimarn's  theories,  it  will  be  seen  that  the  effect 
might  be  attributed  chiefly  to  this  factor,  which  would  have 
the  result  of  increasing  the  number  of  nuclei.  It  can  be 
shown,  however,  that  while  viscosity  counts  for  much,  the 
protective  action  of  the  colloid  is  not  due  to  this  alone,  since 
solutions  of  different  bodies  of  equal  viscosity  give  very  dif- 
ferent results.  There  is  here  a  definitely  selective  action, 
dependent  upon  the  colloid  chemical  character  of  the  medium, 
and  largely  specific  in  respect  of  the  substance  precipitated 
or  dispersed.  Provisionally,  we  shall  regard  this  as  a  capillary 
chemical  or  adsorption  effect,  and  discuss  its  nature  more 
fully  both  experimentally  and  theoretically. 

36 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

Technically,  it  is  well  known  that  different  kinds  of  gelatine 
are  by  no  means  equivalent  for  preparing  silver  halide  emul- 
sions. Without  trenching  on  the  colloid  chemistry  of  gelatine, 
or  on  the  nature  of  the  colloid  condition  of  its  sols  and  gels, 
it  is  to  be  noted  that  the  condition  of  a  given  gelatine  will 
depend  to  a  considerable  extent  upon  its  thermal  history,  and 
also  upon  its  content  of  electrolytes.  The  affinity  of  gelatine 


Silver     Halide     Emulsion 
With  Increasing  Gelatine 


1*34 
Concentrations  in  Normalities 

FIG.  11 

for  water — as  shown  particularly  by  its  absorption  as  gel,  and 
also  by  its  behavior  as  sol — is  increased  greatly  by  small 
amounts  of  acid  and  alkali.  Again,  it  is  affected  by  salts, 
some  of  which  increase,  others  decrease,  its  affinity  for  water. 
In  considering  the  effect  of  additions  upon  a  gelatino-halide 
emulsion,  not  only  the  direct  effect  on  the  silver  halides  must 
be  considered,  but  also  the  indirect  effect,  by  way  of  their 
action  on  the  gelatine. 

We  may  at  this  point  anticipate  the  results  given  in  a 
later  chapter  by  stating  that  while  it  is  very  possible,  even 
probable,  that  in  the  preparation  of  emulsions,  particularly 
in  the  ripening  process,  a  combination  of  some  kind  between 
the  gelatine  and  the  silver  halide  occurs,  we  know  little  or 
nothing  as  to  its  character  or  extent.  We  have  found,  follow- 
ing Eder,1  that  silver  bromide  precipitated  in  gelatine  and 

1  Eder,  J.  M..  1.  c..  Vol.  Ill,  p.  11. 

37 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

centrifuged  out  at  1,000  to  2,000  revolutions  per  minute 
carries  with  it,  after  rapid  washing  with  warm  water,  about 
twTo  per  cent  of  gelatine.  Similar  figures  were  obtained  with 
silver  halide  emulsions.  However,  the  greater  part  of  this  is 
probably  mechanically  retained  and  it  is  most  probable  that 


Silver     Halide     Emulsion 
With  Increasing  Gelatine 


1  Z  3  4 

Concentrations  in  Normalities 

FIG.  12 

the  amount  of  "combined"  gelatine  in  the  ripened  high  speed 
emulsions  is  of  the  order  of  the  dyes  retained  in  sensitizing. 

For  low-speed  and  positive  emulsions  the  nature  of  the 
combination  between  the  gelatine  and  the  silver  halide  is 
probably  even  less  definable.  Nor  do  we  know  whether  or 
not  there  is  a  solution  or  adsorption  of  the  gelatine  as  a  whole, 
or  whether  there  is  a  selective  (preferential)  solution,  sorption 
or  combination  of  amino-acid  anhydrides  which  may  be 
considered  as  the  potential  structure-units  of  the  gelatine 
solution  aggregate,  or  of  protein  derivatives. 

Influence  of  Temperature.  The  general  conclusions  outlined 
here — the  shape  of  the  dispersity-concentration  curve,  etc., 
— are  much  modified  by  change  of  temperature.  By  reducing 
the  amount  of  gelatine  and  raising  the  temperature  to  near  the 
boiling  point,  the  zone  of  minimum  dispersity  is  markedly 
enlarged,  so  that  by  adding  the  first  part  of  the  precipitating 

38 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

silver  solution  drop  by  drop,  thus  forming  (according  to  the 
principles  discussed),  a  nucleus  solution,  then  adding  the 
remainder  relatively  rapidly  and  stirring  well,  a  well  denned 
crystalline  precipitate  is  obtained.  The  principle  is,  of 
course,  familiar  in  analytical  chemistry  in  the  control  of 
precipitations  for  gravimetric  analysis.1 

Colloid  Medium  and  Crystal  Form.  We  must,  however, 
notice  that  the  effect  of  a  colloid  medium  upon  the  dispersity 
of  a  precipitate  is  closely  connected  with  its  effect  upon  the 
form  of  a  crystal.  This  will  be  readily  understood  in  view  of 
the  fact  that  it  affects  not  only  the  condensation  but  the  rate 
of  crystal  growth. 

This  may  be  studied  in  its  most  pronounced  form  when 
crystallization  occurs  at  rest.  Generally,  of  course,  agitation 
tends  to  diminution  of  the  size  of  crystals,  quiescence  to 
increase.  The  object  being  to  obtain  a  uniform,  relatively 
fine-grained  material,  silver  halide  emulsions  are  continu- 
ously and  thoroughly  stirred. 

Studying,  for  maximum  contrast,  the  conditions  of  crystal- 
lization at  rest  with  a  colloid  present  affecting  crystal  growth, 
there  are,  from  one  point  of  view,  the  following  possibilities:2 

a.  Total  inhibition  of  crystallization; 

b.  Suppression  of  some  of  the  lines  of  growth; 

c.  Extension  of  the  crystal  to  abnormal  proportions,  forming  a  compound 
crystal ; 

d.  Gyrating  and  curving  direction  of  growth. 

Of  these,  the  first  need  not  be  considered  here.  The  others 
will  be  discussed  in  order. 

(b)  Suppression  or  repression.    It  is  supposed  that  currents 
are  set  up  to  and  from  growing  crystals — i.  e.,  micro-convec- 
tion currents  due  to  gravity  changes,  as  well,  probably,  as 
convergence  of  diffusion  lines,  which  are  again  affected  by 
the  rate  of  crystallization  and  viscosity.     These  currents  are 
likely  to  become  more  accentuated  and  well  defined  for  a 
medium   at   rest,   and   one   of   high   viscosity.3     Hence,   any 
tendency  to  irregularity  of  growth  will  be  facilitated  in  so  far 
as  these  currents  are  favored.     This  important  question  will 
be  taken  up  later.4 

(c)  Extension    of    the    crystal    to    abnormal    dimensions, 
forming  a  compound  crystal.     The  accompanying  photomi- 

1  Cf.  Brother,  G.  H.,  Suggestions  on  some  common  precipitates.     J.  Amer.  Leather 
Chem.  Assoc.   13:  159.   1918. 

2  Bowman,  J.  H.,  A  study  in  crystallization.     J.  Soc.  Chem.  Ind.  25:  143.  1906. 

3  Viscosity  must  have  a  limiting  or  maximum  influence  here,  for  it  tends  to  decrease 
gravitational  convection  currents. 

«  Cf.  Chapter  IX. 

39 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

crographs  (Figs.  13-16)  of  silver  halide  ripened  in  situ  by 
fuming  gelatino-bromide  plates  with  ammonia  illustrate  both 
(b)  and  (c).  In  some  cases  there  are  filiform  and  dendritic 
structures,  due  largely  to  factors  of  the  type  (b)—  i.  e.,  greater 
rapidity  of  condensation  with  very  imperfect  orientation.  In 
others,  there  is  a  definite  tendency  to  form  an  imperfect 


FIG.  13 
Crystal  aggregate,  magnified  40  diameters 

skeletal  example  of  a  much  larger  compound  crystal,  the 
constituent  crystallites  being  oriented  in  planes  at  definite 
angles  to  each  other. 

In  some  cases,  within  the  same  diffusion  sphere  (or  con- 
densation-halo) it  will  be  seen  that  on  one  side  the  condensation 
tendency  has  prevailed,  giving  a  dendrite,  and  on  the  other, 
the  orientation  tendency,  giving  an  aggregate-skeleton,  or 
compound  crystallo-crystalline  aggregate. 

These  types  of  growth  (b)  and  (c)  at  rest  are  both  compat- 
ible with  what  Bowerman  terms  the  relay  principle, — i.  e.,  a 
growing  point  is  pushed  forward  into  the  supersaturated 
mass  and  becomes  a  new  focus  or  nucleus. 

(d)  Finally,  curving  or  gyrating  occurs  where  the  crystal- 
lizing force  is  so  nearly  balanced  by  the  resistance  to  growth 

40 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

that  this  takes  place  along  the  lines  of  least  resistance.  There 
are  reasons  for  stating  that  in  a  system  at  rest  this  will  generally 
approach  a  logarithmic  spiral. 

Taken  altogether,  the  phenomena  in  a  colloid  gel  (at  rest) 
indicate  that  the  supply,  as  fixed  by  the  diffusion  potentials 
of  the  supersaturated  field,  is  more  or  less  divided  in  distribu- 
tion between  local  condensation  and  uniform  orientation  of 
the  crystallizing  molecules.  There  results,  then,  a  particular 


FIG.  14 
Crystal  aggregate,  magnified  40  diameters 

equilibrium  between  the  former  tendency  to  increased  density 
in  phase  (condensation)  and  the  latter  tendency  to  oriented 
or  ordered  distribution  in  phase  (crystallization  per  se),  which 
is  possible  only  in  systems  at  rest,  where  the  diffusion  sphere 
may  be  large1  (cf.  Fig.  5).  Agitation  destroys  this  condition, 

1  The  principle  here  outlined  is  an  application  of  Gibbs'  theorem  on  the  homology  of 
macro-canonical  and  micro-canonical  ensembles,  as  given  in  Elementary  Principles  of 
Statistical  Mechanics.  On  formation  of  skeletons  and  crystal  growths  in  general,  see  R. 
Brauns,  Chemische  Mineralogie,  p.  130,  and  O.  Lehmann,  Molekular  Physik,  Vol.  I,  p.  337. 
Lehmann's  explanation  of  the  local  concentration  gradients  along  the  lines  of  auto-intensi- 
fication applies  to  the  growth  forms  (dendrites),  but  our  contention  is  that  these  are  char- 
acteristically antithetic  to  the  skeletal  forms,  and  that  this  difference  is  explicable  on  the 
differentiation  pointed  out  by  E.  Riecke  (Ueber  Wechselwirkung  und  Gleichgewicht 
trigonaler  Polysysteme,  Ann.  Physik.  IV.  3:  543.  1900).  Riecke  considers  that  the 
growth  of  a  crystal  depends,  inherently,  on  the  exertion  of  both  attractive  (condensing) 
and  orienting  (ordering)  forces  by  the  original  nucleus  on  the  crystallizing  substance. 
This  subject  will  be  discussed  more  fully  in  a  later  chapter. 

41 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

— i.  e.,  reduces  aggregation-crystallization  to  a  relative  mini- 
mum depending  upon  chance  of  collision  of  crystal  nuclei, 
which  increases  with  total  concentration  of  reactants,  and 
decreases  with  the  viscosity  of  any  colloid  present. 

Actually,  then,  agitation  under  certain  conditions  may 
favor  crystallization,  for  by  eliminating  certain  types  of 
aggregation-crystallization  it  increases  the  chances  of  devel- 
opment of  individual  crystals.  The  system  to  which  the 
crystal  belongs  can  be  supposed  to  be  determined  inherently 
by  its  chemical  constitution,  or,  if  polymorphic,  by  conditions 
of  pressure  and  temperature. 


FIG.  15 

Ammonia  recrystallization  of  silver  bromide,  exfoliatory 
aggregation,  magnified  30  times 

The  facts  just  described  may  be  partially  accounted  for 
by  certain  considerations  of  the  influence  of  capillarity  upon 
crystal  form,  a  topic  which  will  be  discussed  in  a  subsequent 
chapter.  (See  pp.  57  et  seq.) 

2.    EFFECT    OF   MIXTURE    OF    SILVER   HALIDES 

The  Condition  of  Co- precipitated  Silver  Halides.  When 
two  relatively  insoluble  compounds  with  a  common  ion,  such 
as  silver  chloride  and  silver  bromide  or  silver  bromide  and 
silver  iodide,  are  precipitated  together,  the  proportions  in 
which  they  are  formed  in  the  precipitate  depend  upon : 

1.  The  solubilities  of  the  compounds  in  water; 

2.  The  solubilities  of  the  compounds  in  excess  of  the  precipitants; 

42 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

3.  The  relative  proportions  and  absolute  excesses  of  the  precipitants; 

4.  The  possible  formation  of  definite  compounds  in  between  the  precip- 
itated substance. 

These  conditions,  expressed  in  terms  of  the  mass  law, 
determine  true  equilibrium  for  a  given  temperature.1  We 
must,  however,  bear  in  mind  the  possibility  of  the  end-state 


FIG.  16 

Silver  bromide  aggregate,  recrystallized  by  ammonia  fuming. 
Compound  dendritic  and  cubic  aggregate  structure 

in  a  given  case  being  a  "false  equilibrium,"  owing  to  quasi- 
mechanical  adsorption  factors,  etc.,  so  increasing  the  inner 
friction  that  true  equilibrium  is  not  reached.2  There  are 

1  See  F.  W.  Kuster,  Ueber  Gleichgewichtserscheinungen  bei  Fallungsreaktionen.   Zeits 
anorg.  Chem.  19:  81.   1898. 

2  Duhem,  P.,  Traite  elementaire  de  mecanique  chimique,  fondle  sur  la  thermodyna 
mique.     Vol.  I,  p.  4. 

43 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

several  phenomena  in  the  formation  and  ripening  of  silver 
halides  which  point  to  this  as  a  probability.  The  experi- 
mental test  of  false  equilibrium  is  that  different  end-states 
are  reached  on  proceeding  from  opposite  directions.  It  is 
possible  that  the  discontinuity  between  low-speed  and  high- 
speed emulsions  depends  to  some  extent  upon  this  condition. 

An  important  investigation  in  which  the  precipitation 
relations  of  the  silver  halide  pairs,  silver  chloride-silver  bromide 
and  silver  bromide-silver  iodide,  were  examined  is  due  to 
Thiel.1  The  object  of  the  research  was  the  investigation 
of  reversible  electrodes  of  the  second  kind  with  mixed  depolar- 
izers. The  so-called  second  kind  of  galvanic  elements  are 
concentration  cells  in  which  the  electrodes  are  of  the  same 
element,  having  the  same  solution  tension,  but  bathed  in 
solutions  of  a  slightly  soluble  salt  of  the  electrode  initial  in 
equilibrium  with  a  soluble  salt  of  another  metal  with  the  same 
anion.  These  combinations  are  then  reversible  with  respect 
to  the  anion,  and  the  polarization-preventing  salt  of  the  elec- 
trode metal  is  termed  a  depolarizer. 

Thiel  pointed  out  that  elements  of  the  second  type  might 
be  formed  in  which  the  metal  is  surrounded  by  a  depolarizer 
which  is  not  a  single  solid  body  of  constant  valency,  but  a 
homogeneous  mixture  of  the  body  giving  the  anion  with 
another,  thus  being  analogous  to  an  amalgam.  If  silver 
bromide-silver  chloride,  silver  bromide-silver  iodide,  silver 
chloride-silver  iodide  formed  homogeneous  mixtures,  this 
objective  could  be  realized  with  silver.  Conversely,  potential 
measurements  with  such  combinations  give  information  as 
to  the  homogeneity  of  the  mixtures  of  the  co-precipitated 
silver  halides  under  varying  precipitation  conditions;  and 
this  is  naturally  the  consequence  of  present  interest.  Thiel's 
observations  and  results  of  most  importance  in  this  connection 
were  as  follows: 

1.  When  mixtures  of  silver  bromide  and  silver  iodide  were 
precipitated  together,  noteworthy  peculiarities    in  the  color 
were  observed.     While  the  precipitated  silver  bromide  was 
pale  yellow,  and  the  silver  iodide  only  a  slightly  deeper  yellow, 
mixtures  showed  a  much  deeper  color,  varying  from  lemon 
yellow  to  that  of  egg  yolk.     No  direct  connection  between 
the  color  and  composition  could  be  observed,  as  the  color 
obtained  in  mixtures  of  different  proportions  would  often  be 
the  same,  while  it  might  vary  in  equivalent  mixtures; 

2.  In  the  precipitation  of  pure  silver  iodide  the  presence 
of  a  trace  of  free  iodine  was  evident.     To  keep  this  down  as 

1  Thiel,   A.,   Umkehrbare   Elektroden   zweiter   Art   mit   gemischten   Depolarisatoren. 
Zeits.  anorg.  Chem.  24:  1.  1900. 

44 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

much  as  possible,  the  acidity  (from  sulphuric  acid)  was  kept 
as  low  as  was  compatible  with  obtaining  pure  precipitates.1 
The  color  in  the  solution  was  discharged  by  a  few  drops  of 
sodium  thiosulphate. 

From  this  Thiel  at  first  concluded  that  the  color  of  the 
mixtures  might  be  due  to  a  trace  of  free  iodine  in  the  precipi- 
tate. However,  treatment  with  thiosulphate  failed  to  remove 
it,  so  it  is  probable  that  it  depends  in  some  way  upon  the 
condition  of  the  precipitate — presumably  the  dispersity; 

3.  Investigation  of  the  pair  silver  chloride-silver  bromide 
showed  that  they  formed  homogeneous  mixtures  in  all  propor- 
tions— that  is,  have  unlimited  miscibility; 

4.  On  the  other  hand,  silver  bromide-silver  iodide  have' 
only    limited    miscibility.     In    this    case,    silver    bromide    in 
excess  is  able  to  dissolve  silver  iodide  up  to  thirty  per  cent, 
whereas  silver  iodide  is  able  to  dissolve  silver  bromide  only 
up  to  five  per  cent.     Thus,  if  we  have  altogether  ten  millimols 
of  silver  halides  precipitated  and  saturation  for  silver  iodide 
just  reached,  the  precipitate  would  contain  three  parts  silver 
iodide  to  seven  parts  silver  bromide.     If  now  sufficient  potas- 
sium iodide  were  added  to  the  solution  to  form  four  parts 
silver  iodide  at  equilibrium  in  the  precipitate,  the  solid  phase 
would  consist  of  a  saturated  solution  of  silver  iodide  in  silver 
bromide  (2.5  :  5.9)  and  a  saturated  solution  of  silver  bromide 
in  silver  iodide  (0.1  :  1.5); 

5.  On    comparing    the   amounts   of   silver   bromide-silver 
iodide  in  mixed  precipitates  with  the  theoretical  quantities 
given  by  the  solubility  relations,2  it  was  found  that  more  of 
the  silver  than  calculated  was  always  present.     There  is  a 
tendency    to    preferential    precipitation    of    the    less    soluble 
component. 

In  so  far  as  these  results  bear  on  photographic  emulsions 
it  may  be  noticed  that  the  presumption  formerly  was  that 
either  specific  addition  compounds  of  the  silver  bromide  and 
silver  iodide  were  formed  (Eder)  or  that  silver  bromide  and 
silver  iodide  were  miscible  to  an  unlimited  extent.3  Supposing 
the  precipitates  crystalline,  Thiel  remarks  that  the  behavior 
of  silver  chloride-silver  bromide  mixtures  indicates  a  high 
but  not  perfect  degree  of  isomorphism,  while  with  silver 

1  In  precipitating  silver  halide  the  solution  must  be  acid  rather  than  neutral  or  alkaline, 
as  otherwise  contamination  with  silver  oxide  may  ensue.     In  any  case,  the  only  permissible 
alkali  generally  would  be  ammonium  hydroxide,  which  redissolves  silver  oxide.      (Compare 
the  two  methods  for  making  emulsions,  on  p.  27.)     On  the  other  hand,  where  iodides  are 
used  it  is  evident  that  if  free,  strong  acid  (high  hydrogen-ion  dissociation)  is  present,  free 
iodine,  which  is  strongly  adsorbed  by  silver  iodide,  will  be  formed. 

2  Cf.  Thiel,  1.  c.,  p.  60-63. 

3  Bancroft,  W.  D.,  1.  c.,  p.  650. 

45 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

bromide-silver  iodide  mixtures  there  is  either  only  a  low  grade 
of  isomorphism,  or  isodimorphism. 

Although  the  proportion  of  silver  iodide  in  silver  halide 
emulsions  is  well  within  the  range  of  homogeneous  mixture 
(or  solid  solution),  it  is  probably  sufficient  to  markedly  affect 
the  crystalline  habit  and  growth  in  view  of  the  generally 
anomalous  behavior  of  the  silver  bromide-silver  iodide  mix- 
tures of  the  silver  bromide.1 

While  it  is  not  entirely  permissible  to  compare  separations 
from  liquid  melts  with  precipitations  from  supersaturated 
solutions,  yet  a  contingency  for  the  same  substances  is  obvious. 
Thus  it  is  of  interest  to  note  that  Stoltzenberg  and  Huth2 
concluded  from  thermal  analysis  of  fused  silver  halides  that 
all  three  are  capable  of  forming  a  liquid  crystal  phase,  the 
transition  temperature  of  regular  silver  bromide  to  liquid 
crystal  being  259°  C.  Below  the  transition  points  they  show 
considerable  plasticity.  As  against  this,  Tubandt  and  Lorenz3 
conclude,  from  their  studies  of  the  application  of  conductivity 
determinations  to  polymorphy  of  single  compounds  and  to 
the  state  of  binary  salt  mixtures,  that  there  is  no  definite 
evidence  for  a  liquid  crystal  phase  with  the  silver  halides. 
Further,  as  regards  mixtures  of  the  halides,  they  find  that: 

1.  Monkemeyer's4    conclusion    of    unlimited    miscibility 
between  silver  bromide  and  silver  iodide  is  incorrect.     There 
is  only  a  restricted  miscibility  from  both  sides,  with  series  of 
regular  and  hexagonal  mixed  crystals; 

2.  The  saturation  line  of  regular  mixed  crystals  cuts  the 
crystallization  curve  as  the  proportion  80%  silver  bromide- 
20%  silver  iodide.     It  is  probable  that  here  the  combination 
4AgBr  :  Agl  separates.     The  region  between  20  and  100% 
silver  bromide  probably  consists  of  homogeneous  mixtures 
of  this  compound  (4AgBr  :  Agl)  with  silver  bromide.     Thus, 
in  this  region,  no  transition  phenomena  are  observed,  which 
indicates  stability  of  form; 

3.  Silver    iodide-silver    chloride    gives    complete    mixed 
crystals  up  to  90%  silver  chloride,  with  marked  transition 
phenomena; 

4.  The   crystals   from   melts   containing   80%   and    more 
silver  bromide  are  strongly  birefringent; 

1  Cf.   Chapter  IX. 

2  Stoltzenberg,   H.,   and   Huth,    M.   E.,   Ueber  kristallinisch-flussige   Phasen   bei   den 
Monohalogeniden  des  Thalliums  und  Silbers.     Zeits.  physik.  Chem.  71:  641.  1910. 

3  Tubandt,  C.,  and  Lorenz,  F.,  Das  elektrische  Leitvermogen  als  Methode  zur  Bestim- 
mung  des  Zustandsdiagramms  binarer  Salzgemische.     Zeits.  physik.  Chem.  87:  543.  1914. 

4  Monkemeyer,  K.,  Ueber  die  Bildung  von  Mischkrystallen  der  Blei-,  Silber-,  Thallo- 
und  Cupro-halogene  aus  Schmelzfluss.     Neues  Jahrb.  Mineral.  Geol.  22:  1.  1906. 

46 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

5.  It  may  be  concluded  that  the  only  alternative  to  the 
presence  of  the  combination  4AgBr  :  Agl  is  that  a  third  stable 
modification  of  silver  iodide  is  present  in  the  region  80%  - 100% 
silver  iodide,  such  as  that  indicated  by  Tammann1  as  stable  at 
very  high  pressures.  From  the  conductivity  phenomena 
this  is  not  probable. 

Now,  although  we  can  not  infer  directly  that  the  molecular 
state  of  a  solid  phase  from  a  homogeneous  liquid  melt  will  be 
identical  with  a  solid  phase  of  the  same  composition  obtained 
by  precipitation  from  supersaturated  solution,  yet  it  is  very 
probable  that  the  state  of  the  solid  phase  from  the  liquid 
melt  represents  the  equilibrium  condition  to  which  that  from 
the  supersaturated  solution  tends  to  approach.  Hence  it  is 
possible  that  some  of  the  color  anomalies  of  silver  bromide- 
silver  iodide  mixtures  observed  by  Thiel  may  depend  upon 
the  completeness  of  formation  of  the  compound  4AgBr  :  Agl , 
and  that  this  again  may  be  concerned  in  ripening  phenomena. 
It  is  particularly  interesting  to  note  that  birefringence  in  the 
crystals  of  silver  halide  emulsions  is  now  well  established.2 
In  regard  to  the  general  influence  of  a  co-precipitate  like  silver 
iodide  upon  silver  bromide  or  silver  chloride,  Tubandt  and 
Lorenz  remark  that  the  '  'simple  silver  iodide  molecules  may 
transfer  their  oscillation  condition  to  simple  silver  bromide  or 
silver  chloride  molecules  executing  other  but  similar  oscilla- 
tions, and  thus  help  order  them  in  the  same  space  lattice." 

3.    ADDITION   OF    SOLUBILIZING   AGENTS 

The  influence  of  solubilizing  agents  on  the  dispersity  of 
silver  bromide  in  emulsions  need  not  be  discussed  here,  as  it 
consists  in  intensifying  or  otherwise  modifying  the  saturation 
factor  of  von  Weimarn's  theory;  and  also  because  it  will  be 
considered  in  more  detail  from  the  point  of  view  1)  of  ther- 
modynamic  theory  in  the  discussion  of  capillarity  and  crystal 
growth  (Chapter  V) ,  and  2)  of  molecular  theory  in  the  discussion 
of  crystallization  catalysis  (Chapter  IV). 

4.    ADDITION   OF    SOLUBLE   INGREDIENTS 
OTHER   THAN    SILVER   HALIDES 

The  concluding  statement  of  section  2,  with  regard  to 
mixed  silver  halides,  indicating  what  may  be  termed  a  mutual 
induction  effect  in  crystallization,  brings  us  to  the  heart  of 
the  problem  of  photochemical  sensitizing  of  the  silver  halides, 
including  both  ripening  effects  and  the  so-called  optical  sensi- 
tizing by  the  use  of  certain  dyes. 

1  Tammann,    G.,  Das  Zustandsdiagramm    des   Jodsilbers.    Zeits.    physik.    Chem.  75. 
733.   1911. 

2  Cf.  Chapter  X. 

47 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 
CHEMICAL,  OPTICAL   AND   PHASE    SENSITIZERS 

The  original  photochemical  conception  of  sensitizers,  due 
to  Vogel,1  was  that  they  acted  as  absorbents  of  decomposition 
products, — e.  g.,  silver  nitrate  as  absorbent  of  bromine  from 
silver  bromide  gives  silver  -+-  bromine, — a  function,  as  pointed 
out  by  Bancroft,2  similar  to  that  of  depolarizers  electrochem- 
ically,  and  as  such  indicated  by  Grotthus.3 

At  first  gelatine  was  supposed  to  be  superior  to  collodion 
as  a  medium  because  of  higher  halogen  absorption  power. 
But  it  was  pointed  out  by  Luppo-Cramer4  that  the  superiority 
could  not  rest  on  this  property  alone,  and  that  ordinarily 
chemical  sensitizing  of  this  form  probably  plays  only  a  small 
part  in  the  field  of  gelatino-halide  emulsions.  The  discovery 
of  optical  sensitizers  by  Vogel  brought  forward  a  new  photo- 
chemical problem.  Why  should  certain — quite  a  limited 
number — dyes  make  silver  halides  sensitive  to  their  own 
absorption  region?  There  have  been  two  explanations  for 
this.  One  is  that  a  chemical  decomposition  of  the  dye  is 
effected,  which  is  either  extended  to  the  silver  bromide  before 
development,  or  provides  a  nucleus  for  development.  The 
other  supposes  that  the  internal  vibrations  of  the  dye  molecules 
in  absorption  of  light  affect  the  silver  halide  in  the  same  way 
as  its  own  direct  absorption  of  light. 

It  is  evident  that  the  crux  here  is  very  similar  to  that  for 
the  developability  (latent  image  issue)  of  the  silver  halides 
per  se.  This  is  a  region  where  chemical  and  physical  change 
overlap.  The  theory  of  radiation-transformation,  or  radiation 
catalysis,  has  received  powerful  support  from  the  work  of 
Chapman  and  his  collaborators5  on  the  photochemical  induc- 
tion of  chlorine,6  and  has  been  extended  to  chemical  catalysis 
in  general  by  Lewis.7  In  the  case  of  the  silver  halides  and 
dyestuffs  it  has  been  made  more  precisely  applicable  by  the 
work  of  Stark  and  his  collaborators8  on  "latent  fluorescence" 
and  "ultra-violet  fluorescence." 

1  Vogel,  H.  W.,  Handbuch  der  Photographic.     4th  edition,  Vol.  I.,  pp.  193-195. 

2  Bancroft,  W.  D.,  The  electrochemistry  of  light.     J.  Phys.  Chem.  12:  209,  318,  417. 
1908;  and   13:  1,   181,  449,  538.   1909. 

3  Grotthus,  F.  von,  Physisch-chemische  Forschungen. 

4  Luppo-Cramer,  Photographische  Probleme,  1.  c.,  p.  33. 

5  Burgess,  C.  H.,  and  Chapman,  D.  L.,  The  interaction  of  chlorine  and  hydrogen.     J. 
Chem.  Soc.  (Trans.)  89 2:  1399.  1906.     Chapman,  D.  L.,  Chadwick,  S.,  and  Ramsbottom, 
J.E.,  The  chemical  changes  induced  in  gases  submitted  to  the  action  of  ultra-violet  light. 
J.  Chem.  Soc.   (Trans.)   911:  942.   1907.     Chapman,  D.  L.,  and   MacMahon,   P.  S.,  The 
interaction  of  hydrogen  and  chlorine.     J.  Chem.  Soc.  (Trans.)  951:  135.   1909.    Chapman, 
D.  L.,  and  MacMahon,  P.  S.,  The  retarding  effect  of  oxygen  on  the  rate  of  interaction  of 
chlorine  and  hydrogen.     J.  Chem.  Soc.    Trans.)  951:  959.   1909. 

6  See  also  Sheppard,  S.  E.,  Photochemistry. 

7  Lewis,  W.  C.  M.,  Studies  in  Catalysis.     V.     J.  Chem.  Soc.  (Trans.)  109:  796.  1916. 

8  Stark,  J.,  Zur  Energetik  und  Chemie  der  Bandenspektra.     Physik.  Zeits.  9:  85.   1908; 
Steubing,  W.,  Fluoreszenze  und  lichtelektrische  Empfindlichkeit   organischer   Substanzen 
Physik.  Zeits.  9:  493.   1908. 

48 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

Recent  research  on  selective  absorption  and  fluorescence 
has  shown  that  the  absorption  spectrum  and  the  fluorescence 
spectrum  of  a  substance  are  potentially  equivalent,  but  that 
much  of  the  fluorescent  spectrum  is  easily  rendered  "latent" 
owing  to  re-absorption  and  degradation  by  adjacent  layers  of 
the  same  molecules.  The  fluorescence  spectrum  is  excited 
in  its  maximum  extension  and  intensity  for  layers  practically 
one  molecule  deep.  This  is  strikingly  borne  out  by  R.  W. 
Wood's  work1  on  the  resonance  spectra  of  sodium,  potassium, 
etc. 

Again,  fluorescence  is  not  limited  to  the  visible  region,  but 
may  exist  in  both  the  ultra-violet  and  the  infra-red.  Stark's 
theory  is  substantially  that  optical  sensitizing  is  due  to  ultra- 
violet fluorescence  from  a  layer  of  dye  one  molecule  thick,  and 
that  excess  of  dye  interferes  by  excessive  absorption. 

However,  there  is  one  difficulty  in  the  way  of  this  hypoth- 
esis. The  dyes  which  sensitize  are  quite  limited.  Yet  prac- 
tically all  dyes  containing  a  benzene  ring  with  unstable 
auxo-chromes  or  auxo-fluors  should  be  capable  of  ultra-violet 
fluorescence.  It  appears  probable  that  something  more  is 
necessary — namely,  a  marked  and  selective  capacity  for 
sorption  and  solid  solution  by  the  silver  halide.  The  import- 
ance of  this  will  be  more  fully  evident  on  considering,  on  the 
one  hand,  the  theory  of  the  nature  and  genesis  of  the  crystalline 
condition;  and  on  the  other,  the  experimental  researches 
(particularly  those  of  Retgers,  Reinders  and  Marc),  on  the 
influence  of  additions  on  crystallization. 

It  is  agreed  that  a  crystal  is  a  homogeneous  assemblage  of 
ultimate  particles  of  a  substance,  such  that  each  particle  is 
similarly  situated  and  similarly  environed  by  identical  par- 
ticles, and  that  the  typical  distribution  may  be  referred  to  a 
regular  group  of  points  or  space  lattice. 

It  may  be  pointed  out  at  this  stage  that  one  reason  for  the 
importance  of  crystallization  in  the  formation  of  sensitive 
halide  emulsions  is  the  increased  probability  of  the  extension 
by  resonance  of  the  perturbation  (from  light)  of  a  single 
molecular  layer  throughout  the  mass  of  a  uniform  crystal. 

Further,  however,  it  is  not  agreed  as  to  whether  the 
"ultimate  crystalline  particles"  are  molecules  or  associated 
aggregates  of  molecules,  or,  as  indicated  by  X-ray  crystal 
analysis,  whether  crystallization  involves  loss  of  molecular 
individuality.2  In  any  case,  it  appears  to  be  a  manifestation 


1  Wood,  R.  W.,  Researches  in  physical  optics. 

2  Cf.  I.  Langmuir,  1.  c. 


49 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

of  the  same  intra-molecular  force  which  we  term  "chemical 
affinity"  and  which  is  now  identified  with  the  radiation  field 
proper  to  atom  or  molecule.1  A  consistent  theory  of  the  gene- 
sis of  crystallization  based  on  this  view  has  been  suggested  by 
Beckenkamp.2  He  considers  that  crystallization  is  regulated 
by  approach  to  equilibrium  of  the  inner  radiation  fields  of 
atoms  or  molecules,  in  particular  by  very  short  wave-length 
ether  vibrations  (wave-length  fractions  of  molecular  diameter) . 
Interference  between  these  forms  a  system  of  stationary  waves, 
(the  permanence  of  which  would  obviously  depend  upon 
reduction  of  thermal  energy,  e.  g.,  molecular  kinetic  energy), 
the  nodal  points  of  which  by  mutual  attraction  (resonance) 
determine  the  crystallization  of  simple  forms,  twinning,  etc. 
An  analogy  will  make  this  clearer.  It  is  supposed  that  the 
atoms  (or  molecules)  are  marshalled  in  symmetrical  space 
lattices  by  stationary  ether  wave  systems,  just  as  the  particles 
of  talc  or  lycopodium  are  arranged  in  symmetrical  patterns 
by  stationary  sound  wave  systems  in  Lissajou's  experiments. 
As  differing  from  the  analogy,  however,  the  determining 
stationary  wave  system  is  not  external,  but  internal  and 
inherent,  emanating  from  the  atoms  (or  molecules)  themselves. 
In  other  words,  a  crystal  is  a  concrete  manifestation  of  an 
actino-chemical  equilibrium.  It  becomes  conceivable,  then, 
that  if  a  substance  is  to  act  effectively  as  an  optical  sensitizer, 
in  that  its  specific  absorption  is  to  disturb  the  actino-chemical 
equilibrium,  it  must  also  be  able  to  accommodate  itself  to  the 
space  lattice  of  the  material  sensitized.  Now  it  will  be  seen 
later  that,  to  effect  this,  substances  must  either  be  so  consti- 
tutionally similar  as  to  be  more  or  less  isomorphous,  or  be 
capable  of  the  colloid  condition,  wherein  the  tendency  to 
pronounced  crystalline  form  is  a  minimum,  that  to  molecular 
net-works  (gels)  a  maximum. 

The  operation  of  a  sensitizer  may  be  regarded  in  respect 
of  either  wave-length  or  phase.  It  would  be  very  possible  for 
a  crystalline  substance  to  absorb  only  a  limited  amount  of 
light  of  suitable  wave-lengths,  but  of  such  irregular  phase 
ordering  that  the  equilibrium  radiation  field  of  the  crystal 
would  get  only  partially  in  resonance.  We  can  conceive  then 
that  a  non-crystalline  body  in  solid  solution  or  absorbed  would 
increase  the  photochemical  sensitiveness,  acting  as  a  resonance 
complement.  Thus  colloid  silver  acts  as  a  panchromatic 

1  See  Wyckoff,  R.  W.  G.,  The  nature  of  the  forces  between  atoms  and  solids  (J.  Wash. 
Acad.  Sci.  9:  564.   1919.)  particularly  Classification  of  Crystalline  Solids.     "The  crystalline 
state  furnishes  the  greatest  condensation  of  the  fields  about  the  individual  particles  (atoms 
or  molecules,  depending  upon  the  type  of  solid)." 

2  Beckenkamp,   J.,   Der  tetrakishexagonale    oder  oktaedrische   Typus  der   Kristalle. 
Ann.   Physik.     IV.     39:  346.   1912. 

50 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

sensitizer  for  silver  halides.  The  behavior  of  traces  of  calcium, 
bismuth,  etc.,  in  developing  phosphorescence  in  the  alkaline 
earth  sulphides  is  a  similar  case.1  If  the  above  conception 
is  true  it  is  possible  that  silver  iodide  acts  both  as  a  wave- 
length sensitizer  and  as  a  phase-sensitizer  for  silver  bromide, 
as  well  as  acting  as  an  independent  sourc^  of  silver  nuclei  by 
its  direct  photolysis.  However,  these  contributions  are  likely 
to  be  of  less  total  importance  than  its  stabilizing  influence  as  a 
crystallization  buffer  substance  on  the  crystallization  of  co- 
precipitated  silver  bromide. 

It  has  been  shown  that  a  fundamental  relation  exists 
between  photochemical  catalysts,  positive  and  negative,  and 
crystallization  catalysts.  This  relation  may  be  summed  up 
as  follows:  Crystallization  is  a  process  of  approach  to  a 
complete  (static)  equilibrium  of  the  radiation  fields  (chemical 
affinities)  of  the  component  atoms  and  constituent  molecules. 
The  attainment  of  static  equilibrium  may  be  accelerated  or 
retarded  by  alien  substances,  or  crystallization  catalysts, 
which,  from  the  actino-chemical  nature  of  crystallization, 
are  consequently  likely  to  be  also  photochemical  catalysts, 
affecting  the  transformation  and  redistribution  of  incident 
light  energy.  That  is,  they  affect  the  way  in  which  light 
energy  is  redistributed  by  a  molecule  or  aggregate  M.  This 
may  be  represented  thus: 

Fluorescence  }    Distribution 

Phosphorescence  I    of  energy 


Light  energy — M 


Photochemical  change  >  largely 
Photo-electric  effect       |    determined 
Heat  J    by  catalysts 


Cf.  S.  E.  Sheppard,  1.  c.,  p.  400. 


51 


CHAPTER  IV 

Crystallization  Catalysis 

We  can  conveniently  include  all  the  effects  of  additive 
substances  upon  the  crystallization  of  a  new  phase  under  the 
term  crystallization  catalysis.  By  this  we  shall  understand 
both  the  positive  actions  leading  to  fully  developed  crystals 
and  the  negative  ones  retarding  crystallization  and  leading 
to  well  developed  colloids.  It  follows  from  what  has  been  said 
that  the  seat  of  crystallization  catalysis  is  primarily  the 
interface  between  growing  crystal  and  mother  liquor. 

In  the  simplest  case,  positive  catalysis  of  crystallization  is 
effected  by  substances  which  form  more  soluble,  but  readily 
dissociable  compounds  with  the  crystallizing  substance. 
Thus  the  soluble  bromides  and  ammonia  act  in  this  way  to 
silver  bromide.  Substances  forming  very  stable  soluble 
complexes,  such  as  thiosulphates  and  cyanides,  do  not  have 
this  effect.  On  the  contrary,  especially  if  they  form  less 
soluble  stable  complexes,  such  bodies  are  strong  negative 
catalysts,  such  as,  with  silver  bromide,  bodies  like  mercuric 
bromide  or  lead  bromide  (or  other  mercuric  and  lead  salts). 
These  substances,  it  should  be  noted,  produce  strong  congela- 
tion of  silver  halide  hydrosols.  A  more  complex  case  of 
crystallization  catalysis  exists  where  a  consolute  substance 
affects  the  habit  of  the  growing  crystal.  A  striking  example 
is  the  effect  of  urea  upon  the  crystallization  of  sodium  chloride. 
In  water  alone  sodium  chloride  crystallizes  in  cubes,  but  if 
urea  be  added,  octahedra  are  formed.1  An  important  and 
interesting  series  of  investigations  on  crystallization  from 
aqueous  solution  have  been  made  by  Marc  and  collaborators.2 
Marc  found  that,  while  with  some  substances  the  rate  of 
solution  is  equal  to  the  rate  of  crystallization  up  to  the  greatest 
velocities  of  stirring,  there  are  others  which  show  a  great 
difference;  and  with  these  the  rate  of  crystallization  may  be 
only  one-sixteenth  that  of  solution,  indicating  a  slow  change 
practically  independent  of  diffusion  as  to  speed.  In  any  case, 
however,  crystallization  may  be  brought  to  a  standstill  prema- 
turely by  the  presence  of  dyes,  while  the  rate  of  solution  of 
the  crystals  is  either  not  affected  or  only  somewhat  retarded. 
The  taking  up  of  dye  by  the  crystals  in  general  follows  the 

1  Cf.  Retgers,  J.  W.,   Beitrage  zur  Kenntnis  des  Isomorphismus.     V.     Zeits.  physik. 
Chem.  9:  257.   1892. 

2  Marc,  R.,  Ueber  die  Krystallization  aus  wasserigen  Losungen.     Zeits.  physik.  Chem. 
79:  71.   1912. 

52 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

adsorption  formula,  but  shows  the  phenomenon  of  ''saturation" 
— i.  e.,  with  concentrations  above  a  certain  value  there  is  no 
increment  of  adsorption.  Marc  considers  that  the  surfaces 
of  the  crystal  become  saturated,  and  attributes  the  checking 
of  crystal  growth  to  the  slowness  of  diffusion  of  the  dye  in  the 
solid  crystal,  with  which  it  tends  to  form  a  solid  solution. 

Generally,  colloids  are  readily  adsorbed,1  but  not  so 
crystalloids — unless  either  isomorphous  or  chemically  com- 
bining. Between  the  amounts  of  different  substances  which 
can  saturate  a  given  crystal  surface,2  quantitative  relations 
exist  which  are  conserved  for  other  surfaces.  The  saturation 
indicates  that  an  absolute  minimum  of  surface  energy  is 
reached.  Such  saturated  surfaces  have  lost  all  "free"  surface 
energy,  and  therewith  the  capacity  to  act  as  germ  or  catalyst. 

This  last  corollary  perhaps  indicates  a  relation  to  photo- 
graphic solarization,  where  the  halide  grains  progressively 
lose  capacity  to  function  as  a  "germ"  for  chemical  development. 

That  colloidal  silver  can  be  taken  up  by  crystallizing  silver 
halides  was  shown  by  Reinders.3  He  concludes  that  colloidal 
silver  forms  solid  solutions  with  the  silver  halide,  not  simply  a 
surface  adsorption  layer.  The  photo-halides  are  normal 
salts  of  silver  colored  by  small  amounts  of  colloidal  silver,  the 
color  depending  upon  the  dispersity  of  the  latter.  Certain 
dyes,  and  albumenoids  such  as  gelatine,  are  also  absorbed 
by  the  crystallizing  silver  halides,  and  it  is  noteworthy  that 
gelatine  and  similar  colloids  check  or  completely  prevent 
the  taking  up  of  colloidal  silver.  This  is  a  confirmation  of 
Sheppard  and  Mees'  filter  theory  of  the  value  of  gelatine  as 
an  emulsifying  medium.  Colloidal  gold  behaves  similarly 
to  colloidal  silver. 

The  panchromatizing  effect  of  colloidal  silver  is  very 
probably  responsible  for  a  remarkable  panchromatizing 
effect  discovered  by  J.  G.  Capstaff  of  the  Eastman  Re- 
search Laboratory.  Mr.  Capstaff  found  that,  if  an  ordinary 
dry  plate  or  film4  be  bathed  a  short  time  in  a  two  per  cent 
sodium  bisulphite  solution  (NaHSO3),  then  subjected  to 
prolonged  washing  in  faintly  alkaline  water  and  allowed  to 
dry  spontaneously,  it  becomes  more  or  less  panchromatically 
sensitized.  Although  the  result  is  not  yet  capable  of  precise 
control,  it  has  been  found  that  the  extension  of  spectral 

1  Marc,  R.,  Ueber  Absorption  und  gesattigte  Oberflachen.  Zeits.  physik.  Chem.  81:  641. 
1912. 

2  Marc  worked  with  micro-crystals,  so  this  does  not  specify  a  habit  surface. 

3  Reinders,  W.,  Studien  iiber  die  Photohaloide.     Zeits.  physik.  Chem.  77:  213  and  357. 
1911. 

4  Most  of  the  experiments  were  made  with  Eastman  Portrait  Film. 

53 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

sensibility  runs  parallel  with  duration  of  washing  with  ordin- 
ary (hard)  tap  water.  Thus,  only  a  slight  extension  of  sensi- 
bility was  found  after  five  to  fifteen  minutes'  washing;  very 
considerable  after  one  to  five  hours'  washing;  and  after 
twenty-four  to  thirty  hours'  washing,  sensibility  was  extended 
to  nearly  SOO/nfi.  (See  Fig.  17.)  The  time  of  washing  may 


FIG.  17 

Capstaff  panchromatizing  effect,  showing  stages  in  the  development 
of  color  sensitizing  with  time  of  washing  in  hard  water 

be  greatly  shortened,  five  to  ten  minutes  being  sufficient  to  de- 
velop strong  sensitizing  action,  if  the  wash  water  is  made  faintly 
alkaline  with  sodium  carbonate  (Na2CO3).  (See  Fig.  17,  4.) 

The  nature  of  the  sensitizing  action  will  be  evident  from 
Fig.  171  (1,  2  and  3),  which  shows  phases  of  its  progression  to 


Taken  with  a  wedge  spectrograph. 


54 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

full  panchromatic  sensibility.  It  is  evident  that  the  sensitiz- 
ing action  differs  markedly  from  that  conferred  by  sensitizing 
dyes,  since  the  action  commences  not  with  a  new  band,  but  by 
lateral  extension  toward  the  red  end  of  the  usual  sensitivity 
curve.  There  is  at  the  same  time  a  small  decrease  in  the  blue 
sensitiveness,  but  this  is  more  than  compensated  for  by  the 
increase  in  general  sensitiveness;  hence  the  speed  to  white 
light  is  greatly  increased. 

As  regards  chemical  conditions,  those  at  present  evident 
are  as  follows: 

a.  The  effect  can  be  induced  by  sulphurous  acid,  as  well  as  by  acid  bisul- 
phites; it  is  then  due  immediately  to  sulphurous  acid  (H2SO3); 

b.  If  this  is  washed  out  with  distilled  water,  little  or  no  sensitizing  action 
is  observed; 

c.  An  alkaline  after-bath  or  wash  is  necessary  to  develop  the  induced  or 
presensitizing  effect; 

d.  A  very  small  amount  of   soluble  bromide — e.  g.,  potassium  bromide 
(KBr)  at  a  concentration   of  .004  per  cent — in  the  sensitizing  bath, 
is  able  to  kill  the  effect; 

e.  "Chemical   fog"   increases  progressively  with  the    sensitizing  effect 
although  not  reaching  very  high  values — e.  g.,  D  =  0.6  at  the  limit. 

Provisionally,  the  existing  facts  appear  to  be  compatible 
with  the  view  that  a  small  amount  of  reduction  of  ionic  to 
metallic  silver  is  effected  by  or  in  the  presence  of  sulphurous 
acid,  as  a  presensitizing  effect;  that  this  is  inhibited  by  soluble 
bromide;  and  that  the  alkaline  after-bath  is  necessary  to 
peptize  this  silver  to  higher  dispersity,  by  which  the  pan- 
chromatizing  effect  is  fully  developed.1 

In  view  of  the  similarity  of  the  adsorption  phenomena  of 
the  colloidal  metals  and  dyes,  particularly  the  fact  that  silver 
halides  can  be  panchromatically  sensitized  with  colloidal 
silver,  there  seems  a  measure  of  probability  in  the  view  that 
exposure  to  light  produces  a  substance  which  is  itself  capable 
of  accelerating  the  reaction  to  light  of  the  wave-length  in 
question.  Such  a  presensitizing  effect  would  be  an  example 
of  a  specific  auto-catalysis.  However,  we  are  straying  some- 
what from  the  main  theme.  It  is  sufficient  to  point  out 
that  there  is  some  probability  that  in  ripening  a  certain  amount 
of  adsorption  of  gelatine,  or  of  hydrolytic  derivative  of  gelatine, 
occurs.  Also,  there  is  a  possibility  that  a  very  slight  initiation 
of  reduction  occurs,  which,  however,  is  rapidly  succeeded  by 
spontaneous  fogging.  Again,  it  is  equally  possible  that  the 
most  important  fact  in  ripening  is  a  recrystallization,  involving 

1  It  is  interesting  to  note  that  a  visible  yellowish-orange  discoloration  increases  pro- 
gressively with  the  washing  treatment;  this  runs  approximately  parallel  with  the  sensi- 
tizing action. 

55 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

a  purification  of  the  silver  halide.  Bancroft's  theory1  that 
ripening  involves  progress  of  the  gelatino-silver  halide  grain 
to  a  certain  optimum  composition  of  silver  halide-gelatine- 
water,  but  that  therefore  size  of  grain  is  entirely  unimportant, 
would,  according  to  this  view,  be  only  partially  correct.  For 
first,  only  by  progress  from  the  suspensoid  colloid  to  a  micro- 
crystalline  suspension  of  lower  dispersity  could  adsorbed 
free  bromide  be  completely  washed  out.  It  is  a  mistake  to 
suppose  that  any  amount  of  washing  will  completely  remove 
every  trace  of  a  stabilizing  electrolyte  from  a  colloid  precip- 
itate. Several  peculiarities  of  emulsions  are  no  doubt  connected 
with  this. 

Secondly,  any  further  phenomena  of  purification — e.  g., 
degelatinization — can  not  be  independent  of  the  size  of  grain 
in  more  advanced  recrystallization  as  in  negative  emulsions, 
particularly  high-speed  emulsions,  because  purification  and 
increase  of  size  go  largely  together,  as  is  brought  out  by 
Marc's  investigations.  What  seems  most  probable  for  the 
critical  stage,  when  an  emulsion  is  nearing  the  point  of  "going 
over" — i.  e.,  becoming  liable  to  spontaneous  fog — is  that  a 
process  of  degelatinization  of  the  silver  halide  crystal  is  going 
on  pari  passu  with  a  taking  up  of  colloidal  silver  formed  by 
interaction  of  silver  halide  with  decomposition  products  of 
the  gelatine;  for,  as  shown  by  Reinders,  taking  up  gelatine 
excludes  the  taking  up  of  colloidal  silver.  The  point  at  which 
the  protective  effect  of  the  gelatine  is  passed  (due  to  weakening 
by  hydrolysis)  is  the  point  of  going  over. 

This  view  would  seem  in  some  degree  incompatible  with 
Reinders'  observation  that  silver  halides  crystallizing  from 
gelatine  solutions  are  much  more  sensitive — i.  e.,  that  taking 
up  gelatine  increases  sensitivity.  But  it  must  be  reiterated 
that  this  refers  to  photochemical  sensitiveness — i.  e.,  photo- 
lytic  production  of  visible  darkening  or  coloration.  Photo- 
graphic developability  is  a  phenomenon  of  another  order,  in 
which  excess  of  gelatine  in  the  grain  will  impede  the  chemical 
reduction  of  the  grain  by  the  developer  by  lessening  the  contact 
action  of  the  nuclei,  while  presence  of  a  minimum  trace  of 
colloidal  silver  may  lower  the  quota  of  developability  per 
grain  to  be  added  by  exposure  to  light. 

1  Bancroft,  W.  D.,  The  photographic  plate,  1.  c.,  p.  650. 


56 


CHAPTER  V 
Capillarity  and  Crystal  Growth 

The  original  work  in  this  direction  is  due  to  Gibbs.1  From 
his  investigation  of  the  equilibrium  of  heterogeneous  sub- 
stances he  deduced  that  the  forms  which  are  in  equilibrium 
for  crystals  under  the  influence  of  capillary  forces  are  those 
in  which  the  surface  energy  is  at  a  maximum  or  a  minimum. 
Assuming  that  each  crystal  face  has  its  specific  capillarity 
constant,  measured  by  the  work  of  increasing  the  face  by  the 
unit  of  area,  this  deduction  may  be  expressed  by  assuming 
that 

A^H-  A2S2+  A3S3  +    --..AnSn 

is  a  maximum  or  minimum,  the  areas  of  different  faces  being 
denoted  by  Slt  S,,  etc.,  the  capillarity  constants  by  AI,  A2, 
etc.  Gibbs,  however,  qualified  this  theorem  by  the  statement 
that  the  tendency  of  a  crystal  to  take  up  the  form  set  by  this 
capillarity  equilibrium  is  inversely  proportional  to  its  linear 
dimensions.  He  states  that  "On  the  whole,  it  seems  not 
improbable  that  the  form  of  very  minute  crystals  in  equilibrium 
with  solvents  is  principally  determined  ...  by  the 
condition  that 

A^.4-  A2S2  +    ....AnSn 

shall  be  a  minimum  for  the  volume  of  the  crystal, 
but,  as  they  (the  minute  crystals)  grow  (in  a  solvent  no  more 
supersaturated  than  is  necessary  to  make  them  grow  at  all), 
the  deposition  of  new  matter  on  the  different  surfaces  will  be 
determined  more  by  the  nature  (orientation)  of  the  surfaces 
and  less  by  their  size  and  relations  to  the  surrounding 
surfaces."2 

It  is  in  fact  probable  that  the  surface  energy  principle 
ceases  to  be  regulative  for  crystals  of  a  given  substance  above 
a  certain  size — i.  e.,  beyond  a  certain  dispersity — though 
we  do  not  know  whether  this  is  absolute  or  is  relative  to  the 
nature  of  the  substance.  Although  Gibbs  was  the  originator 
of  this  principle,  it  is  better  known  from  the  work  of  Curie3 
and  Wulff.4  Since  it  is  closely  interwoven  with  all  questions 
of  crystal  size  and  growth,  and  with  the  so-called  "Ostwald 

1  Gibbs,  J.  W.,  Scientific  papers,  Vol.  I.,  pp.  320-326. 

2  Gibbs,  J.  W.,  1.  c.,  p.  325,  footnote. 

3  Curie,  P.,  Sur  la  formation  des  cristaux  et  sur  les  constantes  capillaires  de  leur  dif- 
ferentes  faces.     Bull,  frang.  mineral.  8:  145.   1885. 

4  Wulff,  G.,  Zur  Frage  der  Geschwindigkeit  des  Wachstums  und  der  Auflosung  der 
Krystallflachen.     Zeits.  Kryst.  u.  Mineral.  34:  449.   1901. 

57 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

ripening,"  it  is  desirable  to  consider  more  closely  its  conse- 
quences and  the  experimental  results.  Considering  a  crystal 
in  its  mother  liquor,  only  the  surface  energy  is  variable,  and 
its  growth  will  be  in  that  form  for  which  the  total  surface 
energy  is  a  minimum.  Furthermore,  each  surface  must  have 
its  own  specific  capillarity,  otherwise  a  sphere  would  be  formed. 

For  substances  like  silver  bromide  and  silver  chloride 
crystallizing  in  the  regular  system,  the  condition  for  a  right 
quadratic  prism  is  as  follows:  Let  x  be  the  side  of  the  base, 
y  the  height  of  the  prism,  A  the  capillarity  of  the  (equivalent) 
side  faces,  and  B  of  the  base,  then  the  surface  energy  E  = 
4xyA  =  2x*B. 

Since  the  volume  of  the  prism  is  V  =  x2y  =  constant,  we 
have  $>(x2y)  =  o,  and  equilibrium  will  occur  when  £  is  a 
minimum.  The  necessary  condition  for  this  is  A  (xby  -\-  y&x)  -\- 
Bxkx  =  o  for  any  variations  satisfying  V  =  constant, —  i.  e., 
xby  +  2ybx=  o.  This  gives  at  once  Ay=  Bx;  or  x/y  =  A/B, 
which  means  that  the  capillary  constants  of  the  prism  sur- 
faces and  bases  are  inversely  proportional  to  the  lengths  of 
the  sides.  Similar  calculations  can  be  made  for  a  cube  or  an 
octahedron.  A  regular  octahedron  can  occur  only  if  AIOO: 
Am  >  V~J;  a  cube  if  AIOO:  Am  <  I/ Vs. 

Wulff  and  Hilton1  have  reduced  the  principle  of  minimum 
surface  energy  to  another  form,  formulating  a  generalized 
connection  between  the  capillarities  of  the  various  faces  and 
their  distances  from  the  center  of  the  crystal  for  undisturbed 
growth.  This  is  expressed  in  the  following  theorem: 

The  perpendiculars  on  the  faces  of  a  crystal  from  a  certain 
point  within  it  are  proportional  to  the  capillarities  of  the 
faces,  thus: 

H!  :  h2  :  h3  .  .  .  .hn  =  kl  :  k2  :  k3  :  .  .  .  .kn,  where  h  =  the 
perpendiculars,  k  =  the  capillarities,  and  n  the  total  number 
of  faces. 

This  holds  for  22  of  the  32  crystal  classes.  But  for  the 
other  ten  there  are  an  indefinite  number  of  points  equidistant 
from  all  faces  of  the  same  form.  Wulff2  considers  that  his 
work  on  the  rates  of  growth  of  the  faces  of  Mohr's  salt, 
(NH4)2Fe(SO4)2.6H2O,  established  the  theorem,  but  his 
proof  has  been  called  into  question  by  Hilton;3  and  it  has 
been  pointed  out  by  Friedel4  that  Wulff  obtained  the  same 
results  with  crystals  of  different  habits,  thus  showing  that  a 
distorted  crystal  does  not  tend  to  approach  an  ideal  form. 

1  Hilton,  H.,  Mathematical  Crystallography. 

2  Wulff,  G.,  1.  c. 

3  Hilton,  H.,  1.  c.,  p.  105. 

4  Friedel,  G.,  Examen  critique  de  la  thSorie  de  Curie-Wulff  sur  les  formes  crystallines. 
J.  chim.  phys.  11:478.   1913. 

58 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

Marc  and  Ritzel1  reached  the  same  conclusion  from  their 
work,  which  showed  that  there  are  different  velocities  of 
solubility  on  the  octahedral  and  the  cubic  surfaces  of  these 
crystals.  Since  in  the  case  of  silver  bromide  crystals  in 
photographic  emulsions  one  is  dealing  only  with  octahedral 
faces,  the  Gibbs-Curie-Wulff  law  may  be  applied  without 
limitation,  providing  that  no  other  crystallization  conditions 
arise,  and  that  the  crystals  are  obtained  from  very  slightly 
supersaturated  solutions.  Ostwald  considered  that  his  experi- 
ments on  red  and  yellow  mercuric  oxide2  gave  a  quantitative 
proof  that  the  solubility  (equilibrium  condition)  is  a  function 
of  the  size  of  grain,  and  this  was  supported  by  Hulett's  experi- 
ments3 with  gypsum  (CaSO4). 

Hulett  placed  aqueous  solutions  of  gypsum  in  contact 
with  large  gypsum  plates  and  found  that  equilibrium  occurred 
when  the  concentration  reached  15.33  millimols  per  litre. 
Then,  if  very  fine  gypsum  powder  was  added  to  the  saturated 
solution,  the  concentration  increased,  in  one  case  reaching 
18.2  millimols  per  litre.  This  high  solubility  decreased  very 
rapidly  at  first,  then  more  slowly,  until  after  168  hours  the 
concentration  again  became  15.33  millimols. 

A  similar  experiment  with  very  finely  powdered  baryta 
showed  a  sudden  increase  of  about  80  per  cent  in  the  saturation 
concentration,  which,  as  with  gypsum,  decreased  to  the  normal 
amount  after  long  standing. 

The  size  of  the  gypsum  grains  varied  from  0.2  to  OA/JL. 
The  baryta  grains  averaged  about  0.1  AI.  These  dimensions 
are  at  the  limit  of  microscopic  resolving  power,  and  should 
therefore  be  accepted  with  caution. 

The  experiment  with  gypsum  was  further  complicated  by 
the  presence  of  a  monoclinic  °<-dihydrate  and  a  rhombic 
/3-dihydrate,  into  which  the  o<r-dihydrate  passes  over  when 
left  for  a  considerable  time  in  the  concentrated  solution. 
The  solubility  of  the  /3-dihydrate  is  approximately  30  per  cent 
less  than  that  of  the  ^-dihydrate. 

The  results  of  Hulett's  experiments  have  been  mathe- 
matically worked  out  by  Valeton,4  as  follows: 

If  r  is  the  length  of  a  certain  crystal  edge,  the  volume  of  a 
crystal  grain  of  the  form  under  consideration  ur\  the  surface 

1  Marc,   R.,  and   Ritzel,  A.,  Ueber  die  Faktoren,  die  den  Kristallhabitus  bedingen. 
Zeits.  physik.  Chem.  76:  584.  1911.     Cf.  critique  by  Kuessner,  H.,  ibid.  84:  313.   1913; 
and  Ritzel's  reply,  ibid.  86:  106.   1913. 

2  Ostwald,  W.,  Ueber  die  vermeintliche  Isomerie  des  roten  und  gelben  Quecksilberoxyds 
und  die  Oberflachenspannung  fester  Korper.     Zeits.  physik.  Chem.  34:  495.   1900. 

3  Hulett,  G.  A.,  Beziehungen  zwischen  Oberflachenspannung  und  Loslichkeit.     Zeits. 
physik.  Chem.  37:  385.   1901. 

4  Valeton,  J.  J.,  referred  to  by  Gross,   R.,  Sammelkristallization  in   Beziehung  zum 
Atomfeld  der  Kristalle.     Jahrb.  Rad.  u.  Elekt.   15:  270.   1918. 

59 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

ur2,  the  energy  per  cu.  cm.  i  and  the  surface  tension  per  sq. 
cm.  7,  then  the  total  energy  contained  in  a  grain  having  length 
of  side  r  is — 

Er  =  iurs  -f-  7  ur2  . 

Further,  if  g  is  the  specific  gravity  of  the  crystal  and  M 

M 
the  gram-molecular  concentration  of  the  solution,  then  --is 

~l\  /f  £ 

the   molar   volume  and  r  the  number   of   crystal   grains 

gur3 

contained  in  one  mol.     The  total  energy  in  one  mol  is  then 

.  M  Mi 

ur  =    i h   7  -     — 3  wr2  . 

g  g    vr* 

If  one  takes  —   =    K, 

.  M  ,  M 

then       ur  =    i h   y  K  -  -  . 

g  gr 

For  grains  having  the  edge-length  r  =  ^  , 

.  M 
«„  -  *  T   . 

The  difference  u,  —u^  is  the  work  necessary  to  powder 
one  mol  of  infinite  size  to  grains  of  a  size  corresponding  to 
the  edge-length  r. 

Now,  if  the  very  large  grains  are  in  equilibrium  with 
concentration  c^ ,  then  there  is,  according  to  thermodynamic 
conditions  for  equilibrium,  a  concentration  c  for  saturated 
solutions  in  contact  with  the  grain  size  r,  of  which  the  osmotic 
work  in  one  mol  is  increased  by  an  amount  corresponding 
to  the  difference  between  c  and  c^.  This  is  developed  as 
follows: 

In  the  case  of  dilute  solutions  the  osmotic  work  may  be 
represented  approximately  by  the  equation — 

RT  =   £,  where 

p  =   osmotic  pressure,  T  =   absolute  temperature, 
R  =  gas  constant. 

Therefore, 

RTlnci   =    RTlnc^    +   jK  — 

7K         M 

or         Cl    =    c^    .  e  RT  '  "  . 

On  the  basis  of  these  relations  Vale  ton  has  worked  out  a 
diagram  (Fig.  18)  by  substituting  the  pairs  of  values  found  by 

60 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

Hulett  for  grain  sizes  2  and  0.2  and  the  corresponding  concen- 
trations of  15.33  and  18.2  millimols  per  litre.1 

It  will  be  remembered  that  the  conclusion  that 
20  1  solubility  is  a  function  of  the  size  of  grain  is  incor- 

porated in  von  Weimarn's  theory  of  precipitation. 
It  has  also  been  very  generally  accepted  as  apply- 
ing to  the  ripening  of  photographic  silver  halide 
g       \         emulsions.      As  already  stated,  procedure  here  is 
'         broadly  divided  into  (a)  the  boiling  process,  using 
excess  soluble  bromide  in  slightly  acid  solution,  and 
(b)  the  ammonia  process,  carried  out  at  lower  tem- 
peratures.    In  either  case,  a  particular  type  of  sol- 


14 


123456789  10 

RADIUS    OF    PARTICLES    IN    /. 

Fig  18 

vent  for  silver  bromide  is  present,  the  action  of  which  will  be 
considered  specifically.  The  presence  of  solvents  tending  to 
form  complexes  does  not  necessarily  affect  the  argument  as 
to  Ostwald  ripening. 

The  first  to  attempt  a  microscopic  and  semi-quantitative 
survey  of  the  photographic  ripening  process  were  Bellach  and 
Schaum.2  As  a  first  result  Bellach  observed  that  in  certain 
stages  of  ripening,  beside  relatively  shapeless  to  spherical 
grains,  definite  crystalline  polyhedra,  apparently  tetragonal, 
were  present.  As  pointed  out  by  Bancroft,3  this  had  been 
previously  observed  by  Banks4  and  has  been  fully  confirmed 
by  other  observers.5  Bellach  at  first  assumed  that  this 
occurred  only  in  mixed  emulsions,  but  later  found  it  in  pure 
silver  bromide  emulsions  prepared  by  himself.  Crystallization 
was  observed  after  a  certain  time  both  when  the  boiling  process 
was  employed,  and  with  ammonia  ripening. 

At  the  same  time,  the  average  size  of  grain  increased,  the 
photomicrographs  showing  this  to  concur  with  the  disap- 

1  For  a  critique  of  this  work,  see  Gross,  R.,  1.  c. 

2  Bellach,  V.,  Die  Struktur  der  photographischen  Negative,  1.  c. 

3  Bancroft,  W.  D.,  The  photographic  plate,  1.  c. 

4  Banks,  Remarks  in  discussion  of  paper  by  Hurter  and  Driffield  on  the  latent  image. 
Phot.  J.  22:  159.   1898. 

5  Dyer,  Note  under  Emulsionsbereitung.    Jahrb.  Phot.  18:  437.  1904.    Sheppard,  S.  E., 
and  Mees,  C.  E.  K.,  1.  c.,  p.  51.     Liippo-Cramer,  Phot.  Prob.,  1.  c. 

61 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

pearance  of  small  grains.  There  appears  to  be  evidence  that 
for  certain  straight  silver  bromide  emulsions  a  definite  Ostwald 
ripening  occurs.  It  must  be  noticed,  however,  that  in  practice 
variable  amounts  of  silver  iodide  are  also  present,  and  the 
influence  of  this  will  have  to  be  considered  separately.  We 
may  note  that  per  se  it  appears  to  show  little  or  no  ripening 
in  gelatine.1 

EXAMPLE   OF    OSTWALD   RIPENING   WITH   MERCURIC   IODIDE 

An  apparently  well  developed  example  of  Ostwald  ripening 
was  observed  by  us  in  an  experimental  study  of  the  photo- 
chemistry of  mercuric  iodide.  Previous  investigators  (notably 
Luppo-Cramer),  working  with  this  compound  observed  that, 
when  precipitated  in  gelatine,  it  appears  first  as  the  yellow, 
unstable  modification,  which  crystallizes  in  the  rhombic 
system  and  which  passes  over  to  the  stable  red  iodide,  crystal- 
lizing in  the  tetragonal  system,  normally  below  127°  C.,2 
the  transition  temperature.  Luppo-Cramer  tried  various 
colloid  media,  finding  that  with  gum  arabic  the  red  stable 
form  is  immediately  produced — which  indicates  a  lower 
protective  effect.  When  the  iodide  is  precipitated  in  gelatine 
in  presence  of  excess  of  potassium  iodide,  it  appears  first  as  a 
yellow,  very  finely  divided  colloidal  suspension,  which  on 
being  digested  at  70-90°  C.  in  presence  of  excess  of  potassium 
iodide  passes  over  to  the  regular  form. 

The  emulsion  used  in  our  experiments  was  in  general 
prepared  as  follows: 

a.  10  gms.  soft  gelatine   in  400  cc.  H2O 

b.  10  gms.  HgCl2  "   100  cc.  H2O  (hot) 

c.  10  gms.  KI  "     50  cc.  H2O 

d.  20  gms.  hard  gelatine  "     60  cc.  H2O 

Emulsion  H  -  24 

(a)  and  (b)  are  mixed  together  at  60-70°  C.,  then  (c)  is 
added  with  careful  stirring.  The  emulsion  was  ripened  by 
heating  at  70-90°  C.  for  60  to  190  minutes.  Samples  were 
removed  at  definite  intervals  for  centrifugal  analysis.  The 
setting  gelatine  (d)  was  added  before  washing  and  coating. 
The  progressive  change  or  ripening  effected  could  be  followed 
visually  by  the  color  change  from  yellow  through  salmon  pink 
to  deep  red.  At  the  same  time  the  increase  in  size  of  grain 

1  Luppo-Cramer,  Phot.  Prpb.,  1.  c.,  but  this  depends  on  presence  of  excess  of  soluble 
iodide.     From  this  crystallization  occurs  readily. 

2  Reinders,  W.,  Ueber  die  Bildung  und  Umwandlung  der  mischkrystalle  von  Queck- 
silberbromid  und  Quecksilberjodid.     Zeits.  physik.  Chem.  32:  494.   1900. 

62 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

and  probably  also  in  density  was  evident  on  centrifuging 
samples  of  the  emulsion  at  different  stages.  The  following 
table  illustrates  this  (see  Fig.  19) : 

CENTRIFUGE 
NO.  TEMPERATURE      TIME  OF  COOK  TIME  R.  P.  M. 

1  43°  C.  0  5  mins.  1000 

2  78°  C.  5  mins.  5  mins.  1000 

3  75°  C.  15  mins.  5  mins.  1000 

4  85°  C.  40  mins.  5  mins.  1000 

5  88°  C.  60  mins.  5  mins.  1000 

6  90°  C.  80  mins.  10  mins.  1000 

7  90°  C.  100  mins.  10  mins.  1000 

8  90°  C.  120  mins.  10  mins.  1000 

9  90°  C.  140  mins.  10  mins.  1000 
10  90°  C.  140  mins.  10  mins.  1000 

The  progress  of  ripening  is  also  shown  microscopically  in 
an  increase  in  size  of  grain  and  concurrent  disappearance  of 
smaller  grains.  This  is  shown  by  the  accompanying  photo- 
micrographs. (Fig.  20.)  The  changes  are  also  shown  by 
the  centrifugal  separations. 

Here,  then,  there  appears  a  definite  example  of  Ostwald 
ripening,  in  the  sense  of  the  "eating  up"  of  smaller  grains  by 
larger  ones,  fairly  well  formed  tetragonal  octahedra  of  mercuric 
iodide  being  formed.  At  the  same  time  the  emulsion  acquired 
higher  sensitiveness  and  density-giving  power,  although,  of 
course,  still  much  inferior  to  silver  bromide.  It  must  be 
considered,  however,  before  regarding  this  as  establishing  a 
clear  case  of  Ostwald  ripening,  that  another  factor  is  present. 
This  is  the  initial  appearance  of  mercuric  iodide  in  the  yellow 
form,  belonging  to  the  rhombic  system,  and  stable  only  above 
129.5°  C.  The  colloid  gelatine  has  acted  here  as  agent  of 
retarded  transformation.  In  this  connection  it  is  important 
to  note  that  silver  iodide  also  is  polymorphic,  crystallizing 
in  the  hexagonal  form  below  145°  C.,  and  in  the  regular  above 
this  temperature.  There  is,  therefore,  an  obvious  possibility 
that  silver  iodide  precipitated  in  gelatine  or,  generally  speak- 
ing, under  conditions  favoring  retarded  transformation,  may 
occur  initially  in  the  unstable  form,  passing  over,  similarly  to 
mercilric  iodide,  to  the  stable  form  on  digestion.  There  are 
certain  phenomena  in  straight  silver  iodide  emulsions  which 
point  to  this.  Reciprocally,  if  precipitated  with  silver  bromide, 
this  potentiality  may  be  of  importance. 

The  condition  or  mode  of  combination  of  silver  iodide  with 
silver  bromide,  important  per  se,  may,  however,  be  only 
accessory  to  two  other  important  roles  of  this  substance  in 
emulsions.  First,  as  relatively  less  soluble  in  either  excess 
potassium  bromide  or  in  ammonia,  it  will  function  in  recrystal- 

63 


Fig.  19 

Ripening  of  mercuric  iodide  emulsion.     Progressive  accumulation  of  the 
red  stable  form  shown  by  centrifuging 

64 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 


X  1000.     0-5  minutes. 


. 
V    *   •  * 

*  *  « 


X  1000.     160  minutes. 


X  1000.     5-15  minutes. 


X  1000.     120  minutes. 


X  1000.     40  minutes.  X  1000.     60-80  minutes. 

Fig.  20 

Progress  of  ripening,  showing  increase  in  size  of  grain 


65 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

lization  as  a  buffer  substance,  tending  to  conserve  and  regulate 
the  number  of  nuclei.  It  has  been  shown  in  Chapter  I  that 
relatively  foreign  substances  can  act  as  nuclei  for  recrystallizing 
silver  halides,  and  one  function  of  silver  iodide  is  very  probably 
of  this  character.  Another  may  well  consist  in  the  greater 
adsorptive  power  of  silver  iodide,  which  can  be  operative  both 
in  emulsion  preparation  in  the  matter  of  taking  up  other 
sensitizers,  and  in  development  after  exposure  in  affecting 
the  adsorption  of  the  developer.1 

Before  concluding  this  discussion,  it  is  desirable  to  point 
out  that  the  principle  of  minimum  surface  energy  can  lead 
to  other  phenomena  than  Ostwald  ripening  in  recrystallization ; 
that  in  fact  this  is  by  no  means  the  necessary  consequence. 
Since  this  is  of  considerable  importance  in  regard  to  the 
possibility  of  making  fairly  fine  and  uniformly  grained  emul- 
sions of  high  speed  we  shall  include  a  brief  account  of  the 
effect  of  the  principle  of  minimum  surface  energy  upon  the 
variability  of  habit,  or  of  the  preferential  growth  of  certain 
faces  of  the  crystal. 

It  has  been  shown  that  this  principle  necessarily  implies 
different  capillarity  constants  for  different  faces.  This, 
however,  involves  different  solubilities  for  different  faces,  for 
otherwise  it  appears  impossible  to  conceive  how  a  distorted 
crystal  can  assume  the  equilibrium  form  (with  minimum 
surface  energy)  unless  certain  faces  dissolve  while  others 
grow.  The  most  complete  discussion  of  this  experimentally 
still  unsettled  point  is  due  to  Ritzel2  and  Kuessner.3  Ritzel 
applied  Freundlich's  corrected  form  of  Ostwald's  formula  for 
the  solubility  of  small  particles  as  related  to  surface  tension.4 
For  a  substance  crystallizing  in  cubes  the  formula  gives  the 
solubility  of  a  cube  (C  w)  of  length  of  side  A  in  relation  to  that 
of  an  infinitely  extended  cube  as  (reference  being  to  perfectly 
developed  forms) : 

M         4$w 
r      _     r  P  RT    '     Pa 

^W     -  V^W'cr,       * 

where  M  =  Molecular  weight, 

R  =  Gas  constant  (.8316  x  10'8), 

T  =  Absolute  temperature, 

Sw  =  Surface  tension  on  cube  face, 

P  =  Density. 

1  See  Sheppard,  S.  E.,  and  Meyer,  G.,  Chemical  induction  and  photographic  develop- 
ment.    J.  Amer.  Chem.  Soc.  42:  689.  1920;  Phot.  J.  69:  12.  1920. 

2  Ritzel,  A.,  Die  Kristalltracht  des  Chlornatriums  in  ihrer  Abhangigkeit  vom  Losungs- 
mittel.     Zeits.  Kryst.  u.  Mineral.  49:  152.  1911. 

3  Kuessner,  H.,  1.  c. 

4  Freudlich,  H.,  Kapillarchemie,  p.  47. 

66 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

Following  Wulff  and  Hilton,  it  is  more  convenient  to  take  as 
characteristic  parameter  of  the  crystal  not  a  but  the  distance 
of  a  surface  from  a  middle  point,  A  =  a  1  2,  when  the  formula 
becomes 

M         2SW 

r    _  r         e  RT  •  PA 

\-/  ^tf       ""  "        ^-XW  ^       * 

While  for  octahedra,  taking  the  distance  B  from  a  middle 
point,  since  total  surface  0  =  12  V  3  .  B2,  mass  m0  =  P.  4  V  3  .  B3, 

M_        2S<, 

,>  RT  '  PB 

•     & 


where  SG  =  surface  tension  of  octahedral  surface.  Kuessner 
finds  that,  for  equilibrium,  the  parameters  A  and  B  must  be 
proportional  to  the  respective  capillarity  constants.  The 
minimum  of  surface  energy  then  entails,  however,  an  equality 
of  solubilities  of  all  faces,  otherwise  the  combination  could 
not  be  in  equilibrium.  It  is  found,  by  executing  a  cyclic 
process  of  transference  and  equating  work  terms,  that  the 
same  solubilities  must  be  possessed  by 
1  .  A  cube  of  parameter  A  ; 

2.  The  cubo-octahedron,  with  parameter  A  for  the  cube 
face,  and  A.S0/SW  for  the  octahedral  face;  and 

3.  The  octahedron  with  parameter  B  =  A.S0  /  Sw. 

But  this  leads  to  a  contradiction.  It  is  shown  that  under 
given  conditions  only  one  form  can  be  stable,  since  the  mini- 
mum of  surface  energy  can  exist  for  only  one  configuration. 

So 

The  single  stable  form  is  a  cubo-octahedron  if  —  -    is  between 

£>w 

v"s  and  I/  V~3;  a  simple  hexahedron  if  S0  >>  V~3  Sw  ;  a  simple 
octahedron  if  S0  <  Sw  I  V^  ;  and  only  then  is  thermodynamic 
stability  ensured. 

Finally,  it  is  pointed  out  that  a  paradox  results  in  that  the 
stable  form  is  that  with  the  greater  solubility;  for  only  then 
can  the  other  be  transformed  into  it  by  way  of  the  cubo- 
octahedron.  There  is,  however,  no  real  contradiction  here, 
in  that  the  stable,  more  soluble  form  may  dissolve  in  a  com- 
mon solution  and  the  other  less  stable  form  will  grow,  for  this 
must  by  growth  transform  into  the  stable  form,  so  that  the 
total  result  would  be  a  single  crystal  of  the  stable  form.  Hence 
it  follows  that  the  principle  of  Ostwald  ripening,  by  solubility 
decrease  with  size  of  crystal,  must  be  applied  with  caution. 
It  will  be  seen  that  recrystallization  may  well  occur  between 
cubical  and  octahedral  forms  of  a  regular  system  without 
reference  to  growth  of  large  crystals  at  expense  of  small.  If, 

67 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

as  suggested  in  this  paper,  the  importance  of  recrystallization 
in  ripening  is  less  concerned  with  increase  of  size  (size  being 
principally  determined  by  initial  condensation  conditions) 
and  more  with  the  inner  'adjustment  of  composition,  the 
possibilities  indicated  here  are  significant.1 

Equally  relevant  to  the  problem  of  the  scope  and  function 
of  crystallization  in  silver  halide  emulsions  is  the  relation  of 
dispersity  to  the  twinning  of  crystals.  It  has  been  shown 
experimentally  by  Johnsen2  that,  compared  with  simple 
forms,  twin  crystals  are  a  labile  phase.  Pawlow3  has  tried  to 
show  that  the  free  energy  of  twins  is  greater  than  that  of 
single  crystals  of  equal  mass,  but  smaller  than  that  of  two 
simple  crystals  which,  combined,  would  be  of  equal  mass. 
It  will  be  seen  that  twin  forms  might  occur  as  an  intermediate 
stage  between  ultra-microscopic  crystals  and  larger  micro- 
crystals,  or  more  generally,  as  pointed  out  by  Niggli,4  we 
can  make  the  following  statement:  In  a  system  of  definite 
dispersity  (surf ace/ volume),  twins  represent  a  labile  phase 
relative  to  single  crystals;  or,  a  fine  crystalline  precipitate 
consisting  entirely  of  twin  forms  will  be  of  higher  dispersity 
than  a  precipitate  of  single  crystals  of  the  same  individual 
mass. 

Reverting  to  von  Weimarn's  analysis  of  the  crystallization 
process,  it  appears  probable  that  the  genesis  of  twin  forms 
may  be  predicated  as  entirely  determined  in  the  amicroscopic 
stage  of  the  dispersed  phase.  This  conception  is  entirely  in 
harmony  with  the  influence  of  solution  factors  on  twinning, 
regarded  as  operative  by  way  of  surface  forces.  The  influence 
of  these  solution  factors  is  necessarily  greater,  the  higher  the 
dispersity. 

We  have  seen  that  the  initial  formation  of  the  labile 
(yellow)  rhombic  form  of  mercuric  iodide  in  gelatine,  which 
is  stable  only  above  129.5°  C.,  and  which  is  converted  into 
the  stable  (red)  form  on  keeping  the  emulsion  melted,  is  in 
line  with  Ostwald's  law  of  stages.  According  to  this  a  new 
phase  appears  first  in  the  form  involving  the  least  loss  of  free 
energy.  Now  since  the  regular  form  of  silver  iodide  is  stable 

1  The  results  of  the  present  investigation  (Chapter  VIII),  of  the  crystal  forms  of  silver 
bromide  in  normal  gelatine-bromide  emulsions  show  only  octahedral  forms.     Hence,  either 
the  above-noted  factor  would  be  imperative  for  silver  bromide  emulsions,  or  limited  to  the 
submicroscopic   stage.     The   reported   preparation   of   fine-grained   highly   ripened   silver 
bromide  emulsions  in  albumen  is  interesting  in  this  connection.     See  Lehmann,  E.,  and 
Knoche,  P.,  Plate-grain  and  albumen  emulsions.     Brit.  J.  Phot.  61:  759.   1914. 

2  Johnsen,  A.,  Untersuchungen  iiber  Kristallzwillinge  und  deren  Zusammenhang  mit 
anderen  Erscheinungen.     Neues  Jahrb.  Mineral.  Geol.  23:  237.  1907. 

3  Pawlow,  P.,  Ueber  die  Bildung,  das  Gleichgewicht,  und  die  Veranderungen  des  Kris- 
talles  im  isothermen  Medium.     Zeits.  physik.  Chem.   72:  385.   1910. 

4  Niggli,  P.,  Kolloidchemie  und  Zwillingskristalle.     Koll.  Zeits.  10:  268.  1912. 

68 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

only  above  145°  C.,  an  initial  appearance  of  silver  iodide  in 
this  form  would  mean  a  greater  relative  retention  of  free 
energy  than  in  the  case  of  mercuric  iodide,  but  might  tend  to 
happen  more  readily  in  the  presence  of  greater  protection 
afforded  by  co-precipitation  with  excess  silver  bromide. 
Silver  bromide  alone  has  not  been  obtained  in  any  other  than 
the  regular  system.  But  the  tendency  to  assume  a  form 
proper  to  a  relatively  labile  system  might  limit  the  effect  to 
twinning  as  the  immediate  sequence  of  the  suspensoid  stage. 
Indeed,  Mugge1  has  suggested  that  twinning  in  any  case 
indicates  an  accommodation  of  the  crystal  to  a  different  space 
grating,  corresponding  to  an  earlier  or  later  energy  condition. 
Since  this  would  involve  a  condition  of  internal  strain,  it  should 
result  in  an  optical  anomaly,  such  as  birefringence  in  uniaxial 
crystals,  a  phenomenon  which  has  been  shown  to  occur  in 
silver  bromide  emulsion  crystals.2 

It  is  possible  (though  this  awaits  determination),  that  this 
occurrence  of  optical  anomaly,  or,  better,  of  anomalous  optical 
activity,  is  the  very  focus  of  ripening  in  relation  to  speed,  etc. 
If  so,  and  if  it  should  be  found  to  be  connected  with  twinning, 
the  conditions  determining  the  occurrence  and  governing 
the — relatively — temporary  fixation  of  this  labile  stage  merit 
earnest  consideration.3 

Going  back  to  the  broadest  generalization  governing  the 
morphogenesis  of  a  new  phase,  we  have  Ostwald's  law  of 
stages.  Now  there  are  two  ways  in  which  the  law  could 
operate  in  crystallogenesis.  One  form  may  be  stated  thus. 
Every  given  crystal  individual  in  its  growth  (evolution  to 
most  stable  form)  tends  to  pass  through  every  stage  of  its 
possible  range  of  forms;  each  transition  reduces  its  free 
energy,  but  each  transition  is  such  as  to  make  the  necessary 
reduction  of  free  energy  a  minimum  at  each  step.  For  example, 
a  substance  which  normally  crystallizes  in  the  regular  system 
as  rigid  crystals,  but  at  higher  temperatures  and  normal 
pressures  in  the  rhombic,  might  also  be  capable  of  plastic 
and  liquid  crystalline  forms  at  both  higher  temperatures  and 
pressures,  as  asserted  of  silver  iodide.  In  forming  a  new 
phase,  especially  of  a  new  component,  the  individuals  would 
then  pass  through  the  sequence; 

Liquid  7*  liquid  -»   plastic  ->  rhombic  -*•  tetragonal 
droplet       crystal       crystal       crystal          crystal, 

1  Mugge,  O.,Ueber  die  Zwillingsbildung  der  Kristalle.    Fortschr.  Mineral.  1:38.  1911. 

2  The  interpenetration  of  gelatine  and  the  co-crystallization  of  silver  iodide  have  also 
to  be  considered  in  this  connection. 

3  Because  indicating  that  the  intensified  development  and  stabilization  of  this  stage  is 
the  direction  in  which  high  speed  fine-grained  emulsions  must  be  sought.     In  existing 
emulsions,  examined  microscopically,  but  few  instances  of  twinning  were  observed.     But 
investigation  of  the  sub-microscopic  stage  is  lacking. 

69 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

the   latter  stable  stages  being  chronologically   later.      This 
represents  a  linear  evolution. 

On  the  other  hand,  there  is  an  alternative  possibility. 
At  the  initiation  of  the  new  independent  component,  nuclei 
of  every  possible  phase  of  the  new  component  are  formed 
simultaneously,  the  transitions  observed  representing  the 
relative  dominance  of  each  there  present  with  resorption  of 
the  regressive  stages,  and  the  progression  of  form  being  deter- 
mined chiefly  by  factors  influencing  dispersity  in  the  amicro- 
scopic  state.  At  any  relatively  permanent  stage,  all  possible 
forms  are  present,  but  in  quantities  determined  by  conditions 
regulating  dispersity-equilibrium. 

This  view  indicates  that  we  should  generally  observe  and 
study  not  the  apparent  linear  evolution  in  time  of  pseudo- 
individual  crystals,  but  the  mutation  in  space  of  the  collective 
mass.  Where  we  expect  to  see  the  chronologically  sequent 
steps  of  a  linear  evolution  we  really  section  off  displacements 
of  the  mobile  equilibrium  between  simultaneous  forms,  the 
equilibrium  among  which  is  above  all  determined  by  the 
tendency  to  forms  of  minimum  free  surface  energy.1  It  will 
be  seen,  as  an  important  consequence,  that  according  to  this 
view  the  new  component  and  new  phase,  at  their  very  incep- 
tion (in  the  amicroscopic  suspensoid  condition),  must  be 
regarded  as  heterogeneous. 

For  an  average  uniform  degree  of  dispersity  (isopsegmaty) , 
the  individual  particles  will  consist  of  arbitrary  crystalline 
aggregates,  polysynthetic  twins,  twins,  and  single  crystals.2 
From  this  start  progress  to  equilibrium  by  reduction  of  free 
energy  will  lie  in  displacement  in  favor  of  single  crystals, 
aggregates  and  twins  reducing  to  these  as  shown  by  Johnsen's 
experiments. 

CONDITIONS   FAVORING    TWINNING 
AND   OTHER   MULTIPLE   FORMS 

Since  twins  are  not  a  stable  form,  it  is  important  to  consider 
the  conditions  favoring  their  occurrence.  Following  von 
Weimarn's  analysis  and  Johnsen's  specific  experiments,  it  is 
evident  that  their  occurrence  is  primarily  determined  in  the 
amicroscopic  stage,  partly  by  collision  of  ultra-microscopic 
crystals.  Barmhauer3  has  pointed  out  that  in  the  evaporation 

1  With  the  progress  of  dispersimetry,  this  mutation  theory  will  find  a  large  field  in 
metallurgy.     The  isolation  of  single  quasi-individual  crystals,  instructive  as  it  may  be, 
indicates  the  resultant  arrest  of  one  line  of  mutation.     We  require  also  the  mass  resultant. 

2  Together,  of  course,    with  "dissolved"   (molecularly    dispersed)   molecules   (or  the 
mother  phase  ,  liquid  crystallites  and  droplets,  and,  if  another  crystalline  system  is  poss- 
ible, a  duplicate  set  of  aggregates,  etc. 

3  Barmhauer,  H.,  Die  neuere  Entwicklung  der  Krystallographie,  p.  121. 

70 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

of  unsaturated  solutions  of  potassium  sulphate  (which  allows 
slow  crystallization),  only  simple  crystals  are  formed,  whereas 
by  rapid  cooling  of  a  hot  concentrated  solution,  giving  high 
supersaturation  and  rapid  crystallization,  there  are  formed  a 
great  number  of  twins.  Further,  infiltration  of  alcohol  into  a 
cold  concentrated  solution  gives  rise  to  great  numbers  of  the 
most  multifariously  shaped  twins,  and  if  a  cold  concentrated 
solution  thickened  with  gelatine  is  evaporated,  there  is  again  a 
large  production  of  variously  shaped  twins  and  triplets. 

Production  of  twins,  etc.,  is  less  easy  with  less  soluble 
substances,  which  of  course  agrees  with  the  influence  of  a 
higher  degree  of  supersaturation,  and  it  is  obviously  possible 
that  this  would  be  affected  by  the  solubilizing  factors  in 
emulsion-ripening.  A  supersaturated  solution  probably 
already  contains  the  whole  permutation  of  crystal  germs 
in  equilibrium  with  molecularly  dispersed  substance  (dissolved 
molecules),  and  hence  not  growing,  but  contributing  to  the 
"colloidal"  properties  of  such  solutions.  On  rapid  cooling 
or  increased  supersaturation,  the  stages  are  fixed  under  the 
dominance  of  primary  inoculation.  And  here  the  nature  of 
the  medium  and  the  initial  concentration  play  the  chief  role. 
Under  the  foregoing  conditions,  a  special  type  of  twinning 
may  predominate,  corresponding  to  specific  alteration  in 
solubility  of  single  faces,  as  already  noticed.  Preferential 
twinning  on  specific  faces  has  been  frequently  observed  by 
mineralogists. 

The  effect  of  inoculation  was  studied  by  Johnsen  in  relation 
to  degree  of  supersaturation  with  enantiomorphous  crystals 
of  sodium  uranyl  acetate,  with  the  following  results: 

SUPERSATURATION  NO.  OF  INOCULATION  EFFECT 

(1  =  SATURATED)  CRYSTALS  D/L 

L.          D. 

.00  13           29  2.23, 

.03  10           23  2.30 

.09  10           25  2.50 

.14  7           22  3.14 

.20  1           40  40.00 

.26  1           45  45.00 

1.31  1           45  45.00 

1.34  labile  14           11  .79 

1.34  labile  5           20  4.00 

It  will  be  seen  that  the  effect  increases  at  first  with  the 
degree  of  supersaturation,  and  decreases  immediately  when 
the  solution  is  labile.  This  comprises  the  view  of  the  prede- 
termination of  forms  in  the  highly  dispersed  stage. 

In  general,  then,  the  twinning  conditions  are  closely 
related  to  those  conditions  of  separation  of  a  new  phase 

71 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

which  lead  to  colloids,  and  the  control  of  twinned  forms  is 
essentially  a  problem  of  dispersoid  chemistry,  depending 
upon  the  methods  used  to  produce  systems  of  great  dispersity. 

Becke1  has  pointed  out  that  in  general  twinned  crystals 
grow  more  rapidly  than  simple  ones.  This  may  be  attributed 
to  the  multiplication  of  the  force  of  crystallization,  or  rate  of 
growth,  at  the  boundary  where  common  directions  of  growth 
radiate. 

These  conditions  have  also  a  bearing  on  the  formation  of 
pseudomorphs  and  mimetic  twinning.  Increased  super- 
saturation  will  usually  involve  nearer  approach  to  a  transition 
point,  and  consequently,  as  suggested  by  Mugge,  increased 
tendency  to  orientation  in  an  altered  space  lattice. 

The  conception  that,  just  before  crystallization,  solutions 
consist  often  not  only  of  different  particles  of  one  modification 
but  of  polymorphic  particles  of  different  modifications  is  in 
agreement  with  Smits'  theory  of  allotropy.2  He  has  shown 
that,  where  two  modifications  of  a  substance  (e.  g.,  mercuric 
iodide,  silver  iodide)  may  exist,  under  certain  conditions  both 
modifications  are  present  in  a  definite  equilibrium  over  a  wide 
temperature  interval  about  the  transition  point  (compare  p.  62). 
The  transition  point  is  thus  a  point  of  separation  of  a  mixture, 
analogous  to  the  '  'cracking"  temperature  of  an  oil-water 
emulsion. 

SUMMARY 

Our  review  of  the  factors  in  the  preparation  and  ripening 
of  silver  halide  emulsions  thus  returns  to  the  point  of  departure. 
Beginning  with  the  dispersion  theory  of  von  Weimarn,  it 
connects  up  the  peculiarities  of  "slow"  and  "rapid"  emulsions 
with  this  analysis.  But  while  the  slower  positive  emulsions 
remain  short-circuited  in  the  region  of  the  purely  colloidal 
phenomena  of  peptization  and  pectization,  high-speed  emul- 
sions require  a  traverse  of  very  definite  crystallization  phe- 
nomena. In  this  connection  it  is  suggested  that  decreased 
dispersity  and  increased  grain  size  is  determined  chiefly  by 
initial  precipitation  conditions,  and  that  ripening  by  way  of 
recrystallization  depends  mainly  on  elimination  of  adsorbed 
impurities.  It  is  pointed  out  that  degelatinization  favors 
adsorption  of  colloidal  silver,  setting  a  limit  to  ripening. 
The  hypothesis  of  Ostwald  ripening  is  discussed  in  connection 
with  the  law  of  minimum  surface  energy,  and  it  is  shown  that 

1  Becke,  F.,  Ueber  die  Ausbildung  der  Zwillingskristalle.     Fortschr.  Mineral.  1:  68. 
1911. 

2  Smits,  A.,   Eine  neue  Theorie  der  Erscheinung  Allotropie.     Zeits.  physik.   Chem. 
76:  421.   1911. 

72 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

mere  increase  in  size  is  not  the  sole  outcome  of  this,  but  that 
equilibrium  relations  between  different  habits  are  involved. 

At  this  point,  leaving  the  field  of  colloid  chemistry  for 
that  of  crystallography,  the  relation  of  twinning  and  habit 
variation  to  the  initial  conditions  of  dispersity  is  discussed, 
and  it  is  shown  that  conditions  similar  to  those  regulating 
the  colloid  state  determine  the  formation  of  twinned  crystals 
and  anomalies  of  crystallization.  The  relation  of  twinning 
to  intra-crystalline  strain  and  anomalous  optical  activity  is 
pointed  out,  and  the  suggestion  made  that  this  relation  may 
be  of  great  importance  in  the  theory  of  emulsions  and  their 
preparation.  Thereby  the  problem  is  brought  back  into  the 
ambit  of  the  analysis  of  initial  precipitation  and  colloid  chem- 
ical regulation  of  dispersity.  The  gamut  of  silver  halide 
emulsions  from  the  slowest  gas-light  to  ultra-rapid  may  be 
conceived  as  disposed  on  a  helix,  the  axis  of  which  is  this 
colloid  chemical  regulation  of  dispersity — that  is,  ratio  of 
surface  to  volume — since  all  the  auxiliary  factors  of  sensitizers 
and  desensitizers,  of  size  of  grain,  of  individual  crystal  habit 
and  eventual  twinning,  of  optical  anomaly  and  strained  space- 
lattice,  are  dependent  thereon. 

None  the  less,  although  this  dispersoid  theory  envisages 
and  embraces  emulsion  phenomena  in  their  entirety,  collect- 
ively and  distributively,  it  will  be  obvious  from  the  foregoing 
that  its  function  is  limited  to  that  of  a  regulative  principle, 
operating  statistically  through  the  principle  of  minimum 
surface  energy.  The  intimate  relation  between  grain  structure 
and  photographic  properties  is,  however,  fundamentally  a 
matter  of  crystallographic  investigation,  and  as  such  is  dealt 
with  in  the  following  chapters. 

Considering  that  initial  precipitation  conditions  determine 
very  much  the  type  of  emulsion  in  the  case  of  development 
emulsions  precipitated  with  excess  halide,  and  considering 
also  the  ripening  or  after  treatment,  the  following  factors  are 
involved : 

(a)  The  precipitate  is  [(AgBr)x  :  (Agl)y]  m  (KBr)n     (Gel3  :  H2O)0; 

(b)  Silver  bromide-silver  chloride  readily  form  continuous  solid  solutions 
in  all  proportions; 

(c)  Silver  bromide-silver  iodide  form  only  restricted  solid  solutions;  it  is 
probable  that  mixtures  containing  up  to  20%  silver  iodide  contain  the 
compound  four  parts  silver  bromide-one  part  silver  iodide,  or  tend  to 
form  this  compound; 

(d)  Silver  iodide  acts  as  a  crystallization  buffer-substance,  a  brake  on 
speed  of  recrystallization.     In  particular,  remaining  undissolved  by 
the  ripening  agent,  it  tends  to  conserve  the  original  number  of  nuclei, 
and  hence  restrict  increase  in  size  of  grains.     It  can  also  affect  adsorp- 
tion; 

73 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

(e)  Sorption  of  the  soluble  additions  is  higher,  the  higher  the  dispersity; 

(f)  This   sorption,    lowering   reactivity    in    development,  is  not  entirely 
removable  by  washing; 

(g)  It  is  reduced  by  recrystallization  in  ripening; 

(h)  Ripening  in  colloid  silver  halide  emulsions  is  mainly  a  flocculation 
phenomenon; 

(i)     Ripening  in  suspensions  proper  is  mainly  a  recrystallization  process; 

(j)  This  recrystallization  process  increases  homogeneity  of  the  silver 
halide  in  the  grain,  reduces  absorbed  bromide,  and  probably  gelatine; 

(k)  Recrystallization,  in  so  far  as  it  affects  ripening,  is  limited  by  formation 
of  saturated  surfaces,  and  very  probably  by  colloid  silver  formation 
(incipient  reduction).  It  is  not  known  at  what  stage  colloid  silver  is 
formed,  but  it  may  occur  early,  and  thus  afford  nuclei  for  recrystalli- 
zation, as  shown  in  ammonia  development  (Chapter  I)  in  the  case  of 
ammonia-ripened  emulsions; 

(1)  When  the  experimental  conditions  regulating  the  three  primary 
factors  (1)  dispersity-distribution,  (2)  recrystallization,  and  (3) 
sorption,  (both  adsorption  and  desorption),  are  completely  known, 
scientific  control  of  the  characteristic  curve — i.  e.,  of  speed,  latitude, 
and  density — will  be  possible. 

These  results  show  that  a  consideration  of  the  dispersity 
and  distribution  of  the  silver  halide  precipitate  is  insufficient 
to  account  completely  for  all  the  facts.  It  therefore  becomes 
necessary  to  study  intensively  the  crystalline  form  and  habit 
of  the  individual  silver  halide  grains,  and  thus  endeavor  to 
determine  in  what  way  this  factor  of  fundamental  structure 
is  related  to  the  facts  reviewed  in  the  preceding  pages. 


74 


CHAPTER  VI 

Experimental  Study  of  the  Crystallization 
of  Silver  Bromide 

Microscopic  examination  of  emulsions  used  for  sensitive 
photographic  plates  reveals  a  definite  crystalline  structure  of 
at  least  a  large  number  of  the  silver  bromide  grains.  F.  W.  T. 
Krohn,1  who  was  apparently  the  first  to  recognize  this  struc- 
ture, made  his  observations  between  1892  and  1901,  though 
his  conclusions  were  not  published  until  1918.  Banks'  is  the 
first  recorded  observation.2 


FIG.  21 

Special  silver  iodo-bromide  emulsion,  magnified  1350 
diameters 

1  Krohn,  F.  W.  T.,  The  mechanism  of  development  of  the  image  in  a  dry-plate  negative. 
Phot.  J.  58:  193.  1918. 

2  Banks,  E.,  Remarks  in  discussion  of  Hurter  and  Driffield's   paper   on  the  latent 
image.     Phot.  J.  22:  159.  1898. 

75 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

Bellach1  and  Luppo-Cramer2  observed  this  structure 
somewhat  later,  though  their  published  accounts,  which 
appeared  in  1903  and  1907  respectively,  preceded  Krohn's 
description. 

In  Figs.  21  and  22  are  shown  photomicrographs  of  an 
emulsion  (magnified  1350  diameters3),  which  was  prepared 


FIG.  22 

Special  silver  iodo-bromide  emulsion,  between  crossed  nicols. 
Magnification,  1350  diameters 


for  this  special  purpose,  and  which  is  distinguished  by  a 
number  of  relatively  very  large  grains,  the  largest  having  a 
diameter  of  6  -  8^.  Fig.  23  is  a  photomicrograph  of  a 
"Radio-bromide"  emulsion  (magnified  2,500  times),  made  by 
Guilleminot  of  Paris,  and  Fig.  24  shows  a  similar  magnifica- 


1  Bellach,  V.,  1.  c. 

2  Luppo-Cramer,  Photographische  Probleme,  p.  51. 

3  For  description  of  the  apparatus  used  see  pp.  82-83. 


76 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

tion  of  Seed  30  emulsion  prepared  by  the  Eastman  Kodak 
Company. 

It  will  be  noticed  in  all  these  figures  that  the  largest  grains 
are  polygons  with  angles  of  60°  and  120°.  There  is  an  obvious 
tendency  to  round  off  the  edges  and  corners  in  the  small 
grains — a  phenomenon  which  is  repeatedly  observed  in  the 


FIG.  23 

Guilleminot's  Radio-bromide  emulsion, 
magnified  2500  diameters 


formation  of  crystals  in  a  colloid  matrix — so  that  the  smallest 
grains  generally  appear  more  or  less  spherical. 

It  will  also  be  noted  that  there  are  apparently  two  kinds 
of  grains,  those  which  are  clear  and  therefore  absorb  little 
transmitted  light,  and  those  which  appear  nearly  black  and 
therefore  absorb  considerable  transmitted  light.  In  the 
original  negative,  however,  these  "dark"  grains  do  not  show 

77 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

uniform  light  absorption.  Instead,  there  seems  to  be  a  net- 
work of  more  or  less  dark  portions,  the  details  of  which  are 
not  shown  in  the  reproduction.  As  may  be  clearly  seen  in 
Fig.  21,  these  dark  bodies  exhibit  the  same  crystallographic 
habit  as  the  transparent  grains,  so  that,  for  the  present  at 


FIG.  24 
Seed  30  emulsion,  magnified  2500  diameters 

least,  there  is  no  justification  for  the  assumption  that  these 
represent  two  different  substances. 

The  ratio  between  the  number  and  the  size  of  the  round 
and  the  polygonal  grains  and  between  the  clear  and  the  opaque 
grains  is  not  constant,  but  may  vary  considerably  in  the 
different  emulsions.  Thus,  for  instance,  Luppo-Cramer1 
prepared  a  photomicrograph  of  an  emulsion  in  which  there 

1  Luppo-Cramer,  Photographische  Probleme,  p.  54. 

78 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

were  no  well-developed  crystal  forms,  all  the  grains  being 
spherical.  This  must  not  be  interpreted  as  meaning  that 
these  spherical  bodies  are  not  crystals,  however,  for  that  which 
determines  whether  or  not  a  given  body  is  crystalline  is 
structure,  not  habit.  The  typical  polygonal  form  of  a  well 
developed  crystal  is  merely  one  manifestation  of  its  structure. 

The  above-mentioned  emulsions  (and  this  may  be  said  of 
practically  all  highly  sensitive  emulsions),  contain  in  addition 
to  the  silver  bromide  a  certain  quantity  of  silver  iodide,  which 
varies  in  different  emulsions.  But  there  is  never  any  indica- 
tion that  either  the  bromide  or  the  iodide  crystals  are  precip- 
itated alone — i.  e.,  without  an  admixture  of  the  other. 

In  investigating  this  question,  122  photomicrographs  of 
as  many  emulsions,  prepared  in  various  ways  and  magnified 
2,500  times,  were  examined  without  finding  one  instance  of 
separate  precipitations  of  the  iodide  and  of  the  bromide,— 
i.  e.,  of  hexagonal  silver  iodide  and  regular  silver  bromide. 
For  silver  iodide  is  polymorphous  and,  as  the  p.-t.  diagram 
of  Bakhuis-Roozeboom1  shows,  the  stable  phase  at  normal 
temperatures  is  hexagonal,  the  transition  point  into  regular 
silver  iodide  being  about  145°  C.  And  it  is  very  improbable 
that  silver  iodide  for  emulsion  purposes  is  precipitated  at 
145°  C.  or  higher.2  Renwick3  is  of  the  opinion  that  the 
hexagonal  silver  iodide  determines  for  the  most  part  the 
crystalline  form  of  the  silver  iodo-bromide  grains,  and  thus 
forces  the  silver  bromide  to  crystallize  according  to  the  hexa- 
gonal system.  But  this  is  not  in  agreement  with  the  facts, 
as  will  be  shown  in  later  paragraphs. 

Thiel4  has  determined  by  electrical  measurements  that, 
at  25°  C.,  silver  iodide  can  form  solid  solutions  with  silver 
bromide  up  to  30  molar  per  cent.  This  opens  the  question 
as  to  whether  all  the  indications  of  isomorphism  mentioned 
by  Mitscherlich  actually  occur  below  these  limits.  As  a 
means  of  investigating  this,  very  carefully  purified  silver 
iodide  was  dissolved  in  ammonia  (D.  =  0.897)  and  the 
solution  allowed  to  evaporate  at  room  temperature  in  an  open 
vessel.  The  crystalline  precipitate  obtained  showed  micro- 

1  Bakhuis-Roozeboom,  Heterogene  Gleichgewichte,  Vol.  I,  p.  128.      See  also  Gmelin- 
Kraut,  Handbuch  der  anorganischen  Chemie,  Vol.  II,  p.  115. 

2  It  may,  however,  tend  to  occur  initially,  at  high  dispersity,  in  the  unstable  regular 
form,  as  is  the  case  with  mercuric  iodide  precipitated  in  gelatine.     (See  Chapter  V,  p.  62.) 
Being,  in  this  form,  isomorphous  with  silver  bromide,  co-precipitation  with  the  bromide 
would  favor  this  condition.     However,  at  ordinary  temperatures  this  form  would  be  ther- 
modynamically  unstable,  and,  on  any  application  of  heat,  would  tend  to  pass  to  the  stable 
form,  thus  setting  up  strain  in  the  associated  silver  bromide  crystals. 

3  Renwick,  F.  F.,  (Discussion  of  Krohn's  paper).     Phot.  J.  58:  195.   1918. 

4  Thiel,  A.,  1.  c. 

79 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

scopic  tabular  crystals  of  the  regular  system  of  the  form  {100} 
and  the  combination  (ill)  with  {100} ,  which  became  faintly 
brownish  in  the  light.  Thus  at  room  temperature  silver 
iodide  separates  from  ammoniacal  solution  in  a  metastable 
phase. 

Mixtures  of  silver  iodide  and  silver  bromide  which  were 
dissolved  in  concentrated  ammonia  crystallized  in  octahedra 
of  the  regular  system.  For  silver  bromide  alone  the  regular 
forms  {ill}  and  {100}  and  combinations  are  known.  Hence 
it  may  be  said  that  regular  metastable  silver  iodide  can, 
within  certain  limits,  produce  isomorphous  mixtures  with 
regular  stable  silver  bromide. 

Since  the  quantity  of  silver  iodide  in  emulsions  falls  below 
these  limits,  silver  iodide  crystals  of  photographic  emulsions 
may  very  well  belong  not  only  to  the  same  crystal  system,  but 
also  to  the  same  crystal  class  as  silver  bromide.  The  classi- 
fication of  silver  bromide  would  then  be  determinative  for  the 
silver  iodo-bromide  crystals  of  emulsions. 

EARLY   CRYSTALLOGRAPHIC   INVESTIGATIONS 
OF    SILVER   BROMIDE 

All  crystallographic  investigations  of  silver  bromide 
confirm  the  existence  of  regular  silver  bromide.  The  crystals 
of  natural  silver  bromide  (bromyrite)  show  forms  {ill}  and 

{100.} 

Investigators  have  interpreted  their  observations  differ- 
ently, as  shown  in  the  following: 

Roscoe  and  Schorlemmer1  reported  that  silver  bromide 
crystallizes  from  aqueous  solutions  of  hydrobromic  acid  and 
mercuric  nitrate  in  octahedra;  Bellach2  described  "tetrahedral 
forms"  of  silver  bromide;  and  mention  is  made  of  hexagonal 
forms  of  silver  bromide,  as  follows : 

Elsden3  says:  "The  crystals  have  no  influence  upon  polar- 
ized light  when  lying  flat,  but  they  appear  to  be  doubly 
refractive  when  the  rays  pass  obliquely  through  them,  as 
if  they  belong  to  the  hexagonal  system";  Baur4  records  that 
amorphous  silver  bromide,  dissolved  in  concentrated  ammonia, 
is  precipitated  in  hexagonal  tablets  upon  diluting  the  solution 

1  Roscoe,  H.  E.,  and  Schorlemmer,  C.,  Treatise  on  chemistry,  Vol.  II.,  p.  472. 

2  Bellach,  V.,  1.  c. 

3  Elsden,  J.  V.,  On  the  formation  of  a  chemical  compound  of  ammonia  with  silver 
bromide.     Phot.    News   25:  174.      1881. 

4  Baur,  E.,  Silber  in  Abegg's  Handbuch  der  anorganischen  Chemie,   Vol.  II,  Partll, 
p.  684. 

80 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

with  five  parts  of  water;  and  Renwick1  writes:  "Silver  bro- 
mide could  occur  both  in  the  cubic  and  in  the  hexagonal 
crystalline  systems."  Krohn2  also  mentions  hexagonal  silver 
bromide,  but  he  probably  means  by  that  octahedral  silver 
bromide  of  the  regular  system  which  crystallizes  in  hexagons. 
The  only  classification  of  silver  bromide  crystals  is  by 
Groth,3  who  assigns  silver  bromide  crystals  to  the  hexakis- 
octahedral  class  of  the  regular  system.  (See  the  concordance 
of  symmetry  classes,  p.  131.) 

THE   PREPARATION   AND   EXAMINATION    OF    THE 
MICROSCOPE   MOUNTS4 

The  Materials  Used.  The  following  chemicals  were  used 
in  the  precipitations: 

Silver  nitrate  made  by  the  Eastman  Kodak  Company; 

Potassium  bromide  'Analyzed,'  from  Kahlbaum; 

Potassium  bromide  U.  S.  P.  IX  from  Merck; 

Ammonia  (D  =  0.897)  purified  double-distilled,  from  Powers- Weight- 

man-Rosengarten  Co.,  Philadelphia; 

Distilled  water  from  the  laboratory. 

These  substances  are  recognized  as  the  purest  utilized  for 
practical  purposes.  A  special  chemical  analysis  was  not 
undertaken,  because  it  was  believed  that  possible  impurities 
have  no  effect  on  the  classification  of  crystals.  Indeed, 
impurities  are  sometimes  desirable,  since  their  presence  may 
alter  the  free  energy  between  the  crystal  surfaces  and  the 
mother  liquor,  and  new  forms  thus  appear.  Since  the  class 
to  which  a  crystal  belongs  is  determined  by  the  highest  sym- 
metry common  to  the  different  crystals,  it  is  desirable  to  have 
quite  a  large  quantity  of  crystals  of  various  forms.  For  this 
reason  a  large  number  of  crystals  were  precipitated  in  the 
presence  of  various  supplementary  agents,  which,  however, 
led  to  no  different  result  than  that  already  obtained  by  the 
crystallization  of  the  silver  bromide  from  unadulterated 
solutions. 

Preparation  of  Crystals.  A  solution  of  potassium  bromide 
was  added  to  a  silver  nitrate  solution,  the  precipitated  amor- 
phous silver  bromide  was  washed  several  times  in  a  beaker 
with  boiling  distilled  water,  and  the  surface  liquor  removed 
by  decantation.  Finally  the  precipitate  was  washed  on  filter 

1  Renwick,  F.  F.,  1.  c. 

2  Krohn,  F.  W.  T.,  1.  c. 

3  Groth,  P.,  Chemische  Krystallographie,  Vol.  II.,  p.  200. 

4  Higson  has  recently  published  an  article  (Photomicrography  in  photographic  re- 
search, Phot.  J.  69:140.  1920  ,  in  which  a  similar  method  of  preparing  microscope  mounts 
is  described. 

81 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

paper  and  dried.  The  silver  bromide  in  slight  excess  was 
put  in  a  bottle  with  ammonia,  and,  to  obtain  equilibrium, 
was  allowed  to  stand  for  a  week  (being  frequently  shaken) 
at  a  temperature  of  20°  C.  This  stock  solution  was  then 
used  in  the  preparation  of  the  silver  bromide  crystals. 

After  being  washed  the  crystals  were  placed  on  a  slide, 
dried,  covered  with  Canada  balsam  and  a  cover-glass,  and 
heated  in  a  drying  oven  at  70°  C.  for  at  least  twelve  hours. 
If  microscopic  examination  showed  that  new  forms  had 
appeared,  the  precipitation  was  repeated  and  the  new  prep- 
aration heated,  often  as  high  as  90°  C. 

As  a  result  of  this  treatment  the  crystals  are  so  massed 
together  that  it  is  possible  to  observe  and  photograph  only  a 
few  isolated  crystals.  Better  results  are  obtained  if,  after 
washing,  the  crystals  are  suspended  in  a  three  per  cent  water 
solution  of  gelatine  and  then  spread  on  the  slide. 

In  all,  192  slides  of  silver  bromide  alone  and  73  of  silver 
bromide  crystals  precipitated  in  the  presence  of  various 
foreign  substances  were  prepared.  Each  preparation  con- 
tained on  the  average  about  2,000  crystals,  so  that  altogether 

more  than  500,000  crystals  were 
prepared. 

The  best  and  most  interesting 
preparations  were  selected  and  care- 
fully examined  under  the  micro- 
scope. The  slide  under  observation 
was  moved  back  and  forth  in  such 
a  way  as  to  give  the  effect  of  moving 
the  objective  in  the  manner  shown 
in  Fig.  25.  The  distance  between 
FlG-  25  the  two  successive  back  and  forth 

Method  of  examining  the  movements  was  thus  in  every  case 
Sa^sTaS,s°fu±er  t™;  much  smaller  than  the  diameter  of 
microscope.  the  microscope  field. 

Apparatus  used. 

a.  In  the  examination.  The  instrument  used  in  the 
investigation  was  a  large  model  Zeiss  microscope  with  an 
illuminating  attachment  and  a  Bausch  and  Lomb  camera 
mounted  on  a  large  horizontal  standard.  An  arc  lamp  of  12 
amperes  and  110  volts,  with  a  rheostat  and  an  Abbe  aplanatic 
condenser  (N.  A.  =  1 .40)  served  as  the  illuminant. 

For  the  800  diameters  magnification  a  Zeiss  2  mm.  oil- 
immersion  apochromat  (N.  A.  =  1.4)  and  Zeiss  compensating 

82 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

ocular  No.  4  were  used:  for  magnification  of  1350  diameters, 
a  Bausch  and  Lomb  1.9  mm.  oil-immersion  objective 
(N.  A.  =  1.3)  with  Zeiss  compensating  ocular  No.  6;  for 
magnifying  2,500  times,  a  Bausch  and  Lomb  1.9  mm.  oil- 
immersion  objective  (N.  A.  =  1.3)  and  Zeiss  compensating 
ocular  No.  12.  In  every  case  a  Wratten  G  filter  was  used, 
arid  the  degree  of  magnification  was  ascertained  by  means  of 
a  Bausch  and  Lomb  stage  micrometer  ruled  to  10/*  and  lOOAt. 
b.  In  preparing  the  photomicrograph.  Since  silver  bromide 
is  yellow,  and  it  is  desirable  to  show  as  many  details  as  possible 
in  the  photomicrograph,  a  yellow  filter  (Wratten  G)  was 


FIG.  26 

Photochemical  decomposition  on  the  octahedral  surfaces 
of  silver  bromide  crystals.  Magnification,  2500  diameters 

used.  There  is  an  additional  advantage  in  this,  as  the  silver 
bromide  decomposes  very  rapidly  in  the  strong  illumination 
in  the  microscope,  and  shutting  out  the  blue  and  violet  rays 
retards  this  decomposition,  although  it  does  not  entirely 
prevent  it. 

83 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 


Figs.  26  and  27,  which  are  photomicrographs  of  two 
different  crystals  taken  after  less  than  two  minutes'  exposure 
to  unfiltered  light,  show  how  rapidly  decomposition  proceeds 
in  unfiltered  light.  This  photochemical  decomposition  does 
not  take  place  simultaneously  over  the  entire  surface  of  the 
crystal,  but  begins  in  isolated  points  from  which  it  spreads 


FIG.  27 

Photochemical  decomposition  on  the  octahedral  surfaces 
of  silver  bromide  crystals.  Magnification,  2500  diameters 

over  the  whole  crystal  until  the  crystal  disappears  (as  has 
been  observed  by  Lorenz1).  In  this  respect,  therefore,  the 
direct  photochemical  decomposition  of  silver  bromide  crystals 
seems  to  proceed  in  a  manner  comparable  to  the  formation 
of  the  developed  image  as  demonstrated  by  Scheffer2  and 
Hodgson.3 

1  Lorenz,  R.,  Kolloidchemie  und  Photographic.     Koll.  Zeits.  22:  103.  1918. 

2  Scheffer,  W.,  Microscopical  researches  on  the  size  and  distribution  of  the  plate  grains. 
Brit.  J.  Phot.  54:  116.  1907. 

3  Hodgson,  M.  B.,  The  physical  characteristics  of  the  elementary  grains  of  a  photo- 
graphic plate.     J.  Frankl.  Inst.  184:  705.  1917. 

84 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

The  presence  or  absence  of  colloids  such  as  gelatine  does 
not  affect  the  progress  of  this  photochemical  decomposition, 
and  it  will  be  seen  that  most  of  the  photomicrographs  contain 
dark  spots  caused  by  this  decomposition. 

The  use  of  the  Wratten  G  filter  made  it  necessary  to  use 
yellow-sensitive  plates;  and  as  it  was  desirable  to  obtain 
negatives  which  would  intensify  delicate  details,  Wratten 
panchromatic  plates  (M-plates),  were  used  and  developed 
for  2^  minutes  at  a  temperature  of  18-20°  C.,  in  the  following 
developer : 

Metol  2.2    gr. 

Hydroquinone  8.8    gr. 

Sodium  sulphite  4.8    gr. 

Sodium  carbonate          4.8    gr. 

Potassium  bromide        0.88  gr. 

Water  to  lOOO.OOgr. 

(It  was  demonstrated  in  the  physical  section  of  this  labora- 
tory that  it  is  possible  under  these  conditions  to  obtain  a 
degree  of  development  where  7  =  2.4  without  danger  of 
appreciable  fog.) 

SILVER   BROMIDE   CRYSTALS 

Of  the  various  known  solvents  of  silver  bromide,  such  as 
hydrobromic  acid,  potassium  bromide,  mercuric  nitrate, 
ammonia,  etc.,  ammonia  has  been  found  the  most  convenient. 
Not  only  can  the  various  crystal  forms  of  silver  bromide  in 
photographic  emulsions  be  accurately  identified  when  crystal- 
lized from  ammoniacal  solutions,  but  also  a  large  quantity  of 
other  forms  which  are  valuable  for  crystal  determinations. 

METHODS    OF   PREPARATION 

There  are  three  different  methods  for  crystallizing  silver 
bromide  from  ammoniacal  solution:  (a)  by  diluting  the 
solution  with  water;  (b)  by  evaporation  of  the  ammonia; 
(c)  by  cooling  the  solution. 

The  first  method  was  used  by  Elsden  in  his  above-men- 
tioned work  on  silver  bromide;  the  second  was  utilized  by 
Reinders1  for  obtaining  crystallized  photo-chloride;  and  the 
third  was  undertaken  at  our  suggestion  by  Mr.  Schneider. 

None  of  these  methods  gave  uniform  results,  crystals  of 
various  forms  being  always  obtained.  No  rule  could  be 
established  for  the  appearance  of  any  one  crystal  form,  for 
everywhere  were  all  possible  grades  and  transitions.  This 
was  probably  due  to  the  impossibility  of  obtaining  identical 

1  Reinders,  W.,  1.  c. 

85 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 


conditions  throughout  the  solution  during  the  process  of 
crystallization.  In  the  more  concentrated  solutions  crystals 
of  silver  bromide-ammonia  compounds,  which  frequently 
produced  singularly  beautiful  pseudomorphs,  also  appeared. 

The  methods  of  preparation  will  be  taken  up  in  order,  and 
the  silver  bromide-ammonia  pseudomorphs  and  means  of 
detecting  them  will  be  treated  in  a  later  section. 

(a)  Dilution  of  ammoniacal  solution.  The  dilution  was 
made  as  rapidly  as  possible  and  at  room  temperature  (about 
20°  C.).  The  crystals  separated  throughout  the  solution 
and  fell  to  the  bottom.  In  reflected  light  they  showed  a 
lively  play  of  colors,  due  to  the  interference  phenomena  of 
thin  layers.  (The  majority  of  these  crystals  were  laminate.) 

For  the  various  conditions  under  which  the  precipitation 
was  made  and  the  results  obtained,  see  the  following  table: 


Original  Solution 


%  NH3 

•-                 .L/11UUOI1 

%,  AgBr       with  H2O 

29.4 

0.4           1:2 

29.4 

0.4 

:21A 

29.4 

0.4 

:3 

29.4 

0.4 

:5 

29.4 

0.4 

:10 

23.5 

0.4 

•2 

23.5 

0.4 

:5 

23.5 

0.4 

:10 

20.5 

0.34         1:5 

20.5 

0.34         1:10 

17.6 

0.26         1:10 

14.7 

0.25          1:10 

Numbers  of 

Special         Well-defined  Preparations 
P'orms           Dimensions      Kxamined 

d,  s,  p,  1          1,2,3 

2 

d,p, 

1,  2,  3 

3 

2   3 

2 

2,3 

8 

2,3 

11 

2,3 

3 

2,  3 

3 

2,  3 

6 

2,  3 

3 

2,  3 

3 

2,  3 

3 

2,  3 

3 

s    =  skeleton 

1    ==  needle 

2    =  plate 

3    =   ordinary 

crystal 

Crystal 
Faces 

O,  C 

o 
o 
o 
o 
o 
o 
o 
o 
o 
o 
o 


0  =  octahedron 
C  =  cube 

1  =  lamelliform 

p  =  pseudomorph 

d  =  dendrite 

As  is  evident  from  the  above,  octahedra  predominate. 
Only  the  first  preparation  contained  a  few  cubes,  and  they 
were  very  irregular.  The  first  preparation  also  showed  the 
greatest  variety  of  crystal  forms,  which  is  readily  compre- 
hensible, since  it  was  impossible  to  obtain  identical  crystal- 
lization conditions  throughout  the  solution  because  of 
concentration  changes  due  to  the  evaporation  of  the  ammonia. 
In  order  to  have  the  conditions  as  nearly  uniform  as  possible, 
however,  the  crystallization  was  carried  out  in  closed  vessels. 

The  dilutions  indicated  in  the  table  are  about  the  limits 
within  which  one  can  obtain  well-developed  crystals. 

(b)  Evaporation  of  ammonia.  Eleven  different  concentra- 
tions of  ammonia  were  prepared,  each  being  diluted  with  ten 

86 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 


per  cent  more  water  than  the  preceding  one.  They  were 
treated  with  a  slight  excess  of  silver  bromide  and  left  in  closed 
vessels  for  a  week.  When  equilibrium  was  established,  several 
drops  of  the  solution  were  put  on  a  slide,  and  the  ammonia 
allowed  to  evaporate.  Then  the  crystals  were  heated.  In 
this  way  thirty-five  slides  were  prepared.  The  highest  con- 
centrations gave  pseudomorphs  in  addition  to  octahedra  in 
the  form  of  the  usual  crystals  and  plates.  Also,  there  were 
the  same  dendritic  and  lamellate-formations  as  in  the  diluted 
ammoniacal  solution. 

(c)  Cooling  the  ammoniacal  solution.  A  given  quantity 
of  ammoniacal  silver  bromide  solution  was  put  in  a  bottle 
and  enough  boiling  distilled  water  added  to  fill  the  bottle. 
It  was  then  sealed  so  that  no  air  would  be  in  contact  with  the 
liquid.  The  cooling  was  accomplished  by  means  of  a  stream 
of  ice-cold  water,  and  the  rate  of  cooling  was  regulated  by 
using  bottles  of  different  volumes.  Too  rapid  cooling  produced 
too  small  crystals,  and  too  slow  cooling  gave  only  well-devel- 
oped crystals.  Very  good  results  were  obtained  by  using  a 
50  cc.  bottle  and  a  10  cc.  and  a  14  cc.  pipette.  Crystals 
obtained  under  these  conditions  showed  the  greatest  variability 
of  forms, — i.  e.,  skeletons,  lamellate-formations  on  the  crystal 
surfaces,  etc. 

The  various  <3j|nditions  under  which  the  crystallization 
was  carried  out  anwven  in  the  table  below : 


Solution                 Degree  of 
%  NH3  %  AgBr      Cooling  (°C.) 

Rapidity      Crystal      I 
of  Cooling       Faces 

Spt 
Foi 

0. 

6 

95°- 

4° 

C 

96°- 

5° 

1. 

18     0.01 

95°- 

22° 

a,  b,  c       r,  c,  o 

S, 

1. 

76     0.02(-) 

95°- 

5° 

a,  b           c,  o 

S, 

94°- 

6° 

2. 

35     0.02(  +  ) 

95°- 

22° 

a,  b,  c       o,  p 

96°- 

5° 

2. 

94     0  .  03 

95°- 

21° 

a,  b,  c       o,  p 

95°- 

4° 

5. 

88     0.06 

95°  - 

20° 

a,  b,  c       o 

a  = 

vessel  of  50  cc. 

0     = 

octahedra 

b  = 

vessel  of  10  cc. 

c   = 

cubes 

c   = 

slow  cooling  at 

r   = 

rhombic  dodecahedra 

room  temperature 

P  = 

pentagonal  dodecahedra 

in  a  50  cc.  bottl 

e 

skeletons 

i  = 

lamellate-formations 

Forms    Dimensions 


s,  1 


Numbers 
of  Prepar- 
ations 

1 

21 
12 


35 


1,  2,  3 
1,  2,3 


1,2,3 
2,  3 


1  =  needles 

2  =  plates 

3  =  ordinary  crystals 


SILVER   BROMIDE-AMMONIA   COMPLEXES 

According  to  Bodlander  and  Fittig,1  silver  bromide  in 
ammoniacal  solution  is  present  as  the  complex  compound 
Ag(NH3)2Br/,  in  which  Ag(NH3)2  is  the  complex  cation. 

1   Bodlander,  G.,  and  Fittig,  R.,  Das    Verhalten    von  Molekularverbindungen  bei  der 
Auflosung.     Zeits.  physik.  Chem.  39:  597.  1902. 

87 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

These  silver  bromide-ammonia  compounds  separate  from 
solution  in  a  solid  phase,  the  following  compounds  being 
produced:*  Ag2(NH3)3Br2;  AgNH3Br;  Ag(NH3)3Br.  There- 
fore, in  the  crystallization  of  silver  bromide  from  ammoniacal 
solutions,  it  is  not  impossible  to  obtain  a  complex  compound 
which  may  lead  to  entirely  erroneous  results.  But  Bodlander 
has  shown  that  these  complex  compounds  are  very  unstable, 
and  in  contact  with  air  or  water  dissociate  into  their  constit- 
uents, thus  forming  pseudomorphic  forms  of  silver  bromide. 
The  following  characteristics  were  tested  as  to  their 
reliability  for  distinguishing  the  silver  bromide  from  the 
ammonia  compound : 

AgBr  Agn(NHz)mBm 

1.  Yellow.  1.  Colorless. 

2.  Sensitive  to  light.  2.   Insensitive  to  light. 

3.  Unchanged  in  ammonia-free  3.  Becomes  turbid  in  ammonia-free 
water.  water. 

4.  Microscopically  unchanged  after  4.  Becomes  opaque  microscopically 
being  heated  to  70°  C.  after  being  heated  to  70°  C. 

5.  Shows  simple  refraction  between  5.  Shows  double  refraction  between 
crossed  nicols.  crossed  nicols. 

1.  The  yellow  color  of  silver  bromide  crystals  is  so  intense 
that  it  can  be  perceived  even  in  crystals  of  +1^  in  diameter, 
and  it  is,  therefore,  a  useful  means  of  distinguishing  silver 
bromide  from  the  colorless  ammonia-compounds. 

2.  The  complex  silver  bromide-ammonia  compounds  are 
unchanged   after  being  in   the  sunshine  in   the  presence  of 
ammonia  for  a  day.    As  soon  as  the  ammonia-pressure  becomes 
too   low,    however,    a   photochemical    decomposition   of   the 
disengaged  silver  bromide  sets  in.     This  gives  the  impression 
that  the  complex  compound  is  sensitive  to  light,  as  the  light- 
decomposed   ammonia  compound   is  very  similar   to   photo- 
chemically  decomposed  silver  bromide.     Therefore,  this  test 
is  not  recommended  for  practical  purposes. 

3  and  4.  These  tests,  the  effect  of  ammonia-free  water 
and  the  effect  of  heat,  may  be  combined.  First,  the  crystals 
are  well  washed  with  distilled  water;  if  they  remain  unchanged 
during  the  wrashing  (the  ammonia-compound  becomes  turbid), 
they  are  heated  to  70°  C.,  or  even  higher,  for  at  least  twelve 
hours.  In  this  heating  the  pseudomorphic  crystals  become 
opaque,  due  to  loss  of  ammonia,  and  may  therefore  be  readily 
detected,  as  the  silver  bromide  crystals  are  unaffected. 

5.  The  double  refraction  of  light  by  the  various  silver 
bromide-ammonia  compounds  has  not  been  definitely  estab- 

1  Ephraim,  F.,  1.  c. 

88 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

lished.  Elsden1  examined  only  one  of  the  complexes — which 
one  is  not  known — and  found  that  it  belonged  to  the  tetragonal 
system.  Hence  too  little  is  known  regarding  this  test  to 
consider  it  reliable. 

SUMMARY    OF    OBSERVATIONS 

Crystal  forms  observed.  In  the  various  preparations 
the  forms  observed  were  as  follows:  {ill1/,  {HO/,  {100} , 
(210),  of  which  {ill}  and  {lOOJ  appeared  as  single  crystals, 
and  of  which  the  following  combinations  were  found:  /I  111  + 
(100);  {111}  +  {110};  {111}  +  {210}. 

The  rhombic  dodecahedra,  which  were  seldom  seen, 
occurred  only  as  small  strips  on  the  edges  of  the  octahedra. 

Special  forms  observed.  Among  the  forms  observed, 
needles,  plates,  "dark"  grains  and  pentagonal  dodecahedra 
merit  special  mention. 

Silver  bromide  needles.  In  a  microscopic  study  there  was 
found  a  developed  and  fixed  photographic  plate  which  under 
the  microscope  showed  very  remarkable  developed  grains  in 
the  form  of  needles.  Figs.  28  and  29  show  two  such  grains, 
magnified  2,500  times,  which  measured  respectively  21/*  and 
12 /*  in  length. 


FIG.  28 

Needle-shaped  grain  occurring 
in  a  silver  iodo-bromide  emul- 
sion; enlarged  2500  diameters. 


FIG.  29 

Needle-shaped  grain  occurring 
in  a  silver  iodo-bromide  emul- 
sion; enlarged  2500  diameters. 


The  usual  developed  grains  of  photographic  plates  are 
somewhat  larger  than  the  original  silver  iodo-bromide  crystals, 
and  generally  have  a  different  form.  (A  photomicrograph, 


1  Elsden,  J.  V.,  I.e. 


89 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 


published  by  M.  B.  Hodgson,1  shows  developed  grains  which 
retain  the  form  of  the  original  crystals  only  in  those  cases 
where  the  crystals  were  very  large.  The  change  in  form  is  so 
slight  in  proportion  to  the  size  of  the  grains  that  the  form  of 
the  crystal  is  but  little  affected.) 

When  there  is  an  accumulation  of  silver  iodo-bromide 
crystals  in  the  emulsion,  or  when  the  distance  between  the 
crystals  is  very  small,  the  developed  grains  coalesce,  which 


FIG.  30 

Crystalline  needles  in  a  silver  iodo-bromide  emulsion, 
enlarged  2500  diameters 

makes  them  appear  very  much  larger  than  the  original  crystals. 
The  needles  shown  in  Figs.  28  and  29  may  be  linear  aggre- 
gations of  silver  iodo-bromide  crystals,  or  may  be  developed 
from  small  needles.  * 

A  microscopic  examination  of  the  original  emulsion  showed 
various  needle-shaped  crystals,  some  of  which  were  photo- 
graphed. (Figs.  30  and  31.)  However,  needles  were  found 


Hodgson,  M.  B.,  1.  c. 


90 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

not  only  in  this  emulsion  but  also  in  a  large  number  of  com- 
merciaf  plates,  though  in  the  latter  the  needles  were  not  of 
such  unusual  size.1  Needles  were  also  found  in  emulsions 
containing  silver  bromide  alone.  Between  crossed  nicols 
these  needles  show  exactly  the  same  optical  behavior  as  the 
other  grains.  This  observation  is  not  new,  as  Luppo-Cramer2 
published  a  photomicrograph  of  an  emulsion  needle  in  1907.3 


FIG.  31 

Crystalline  needles  in  a  silver  iodo-bromide  emulsion, 
enlarged  2500  diameters 

To  make  a  more  accurate  determination  of  the  crystalline 
form  of  these  needles,  various  crystalline  precipitates  were 
examined  microscopically  and  needles  were  found  which 
showed  not  only  octahedral  faces  (Fig.  32)  but  cubical  faces 
as  well.4  Where  there  were  cubic  faces,  the  combination 

1  The  lengths  of  the  needles  in  the  different  emulsions  varied  from  3 /A  to  25  U . 

2  Luppo-Cramer,  Photographische  Probleme,  p.  49. 

3  Wallace  also  refers  to  the  presence  of  "spicular  crystals"  in  certain  emulsions. 

4  There   was    abundant    needle-formation    in  a    preparation  which  contained  0.05% 
aluminium  bromide,  and  good  needle-formation  in  one  to  which  0.5%  strontium  bromide 
had  been  added. 

91 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

111)  +  {100}  frequently  appeared  (Fig.  33).  Needles 
were  also  obtained  by  subliming  silver  bromide,  but  their 
faces  could  not  be  determined  on  account  of  the  rounded 
corners.  Octahedral  needles  were  also  found  in  one  of  the 
above-mentioned  preparations  of  silver  bromide  from  potas- 
sium bromide  solution.  The  needles  are,  therefore,  not  unique 


, 


FIGS.  32  and  33 

An  octahedral  and  a  cubic  needle  of  silver  bromide,  precipitated 

from  ammonical  solution.    Magnification  of  2500  and 

800  diameters,  respectively. 

modifications  of  silver  bromide,  but  must  be  regarded  as  the 
result  of  a  special  development  of  the  silver  bromide  crystals. 

Silver  bromide  plates.  It  was  to  be  expected  that,  in  addi- 
tion to  the  ordinary  silver  bromide  crystals,  developed  more 
or  less  in  accordance  with  the  three  co-ordinates,  and  the 
needles,  which  show  a  marked  growth  in  only  one  direction, 
plates,  or  crystals  which  develop  in  two  directions,  would 
also  appear.  This  was  true  in  most  cases  where  the  silver 
bromide  was  crystallized  from  ammoniacal  potassium  bromide 
solution.  There  were  relatively  few  instances  where  the 
crystals  developed  well  in  all  three  directions. 

That  most  of  the  crystals  are  plates  may  be  shown  as 
follows : 

1.  By  focusing  down  with  a  2  mm.  Zeiss  oil-immersion 
apochromatic  objective  (N.  A.  =  1.4)  and  compensating 
ocular  No.  12  with  the  diaphragm  of  the  condenser 
(N.  A.  =  1.4)  wide  open.  The  small  depth  of  focus  of  this 
system  made  it  possible  to  obtain  a  very  sharp  focus  on  the 
thin  surface  of  the  object.  The  crystals  well  developed  in 
three  directions  were  distinguished  in  that  they  could  be 
sharply  seen  in  more  than  one  focus.  Plates,  on  the  contrary, 
appeared  suddenly  sharp  and  distinct,  and  vanished  almost 
immediately  on  changing  the  focus; 

92 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

2.  When  the  crystals  are  in  Canada  balsam,  one  may  turn 
them  over  and  look  down  at  right  angles  to  the  edges  of  the 
crystals,  most  of  which  are  thinner  than  one  ^; 

3.  The  lively  play  of  colors  in  the  silver  bromide  crystals 
in  reflected  light  and  the  pale  colors  in  transmitted  light,  in 
which  gray  of  the  first  order  in  Newton's  color  series  was  often 
identified,  is  explainable  only  by  the  interference  phenomena 
of  thin  layers. 

How  thin  these  crystals  may  be  is  shown  by  the  fact  that, 
according  to  the  well-known  formula, 

"air         _  XAgBr      (where  n   =    refractive  index, 
"AgBr          Tair~  ^   =    wave-length  of  light), 

the  wave-length  in  silver  bromide  is  less  than  half  as  long  as 
the  wave-length  of  the  corresponding  colors  in  the  air.  Now, 
if  it  is  remembered  that  air  layers  0.3^  thick  can  produce 
marked  interference  colors,  it  may  be  readily  understood 
that  silver  bromide  plates  0.13^  thick  can  produce  the  same 
effect.  (These  facts  first  directed  our  attention  to  plate- 
formation  in  photographic  emulsions.)  Koch  and  du  Prel1 
stated  that  whether  the  silver  bromide  crystals  in  photo- 
graphic emulsions  are  plate-shaped  or  tetrahedral  is  yet  to 
be  determined. 

After  this  form  had  been  verified,  Krohn's  article2  was 
published  (in  1918).  In  this  article  Krohn  stated  that  previous 
to  1901  he  had  been  able  to  observe  emulsion  grains  from  all 
directions  because  of  their  Brownian  movement,  and  had 
come  to  the  following  conclusion :  "The  crystals  are  lammellar 
and  almost  hexagonal  and  are  probably  imperfectly  developed 
octahedra  such  as  one  gets  with  chrome  alum  crystallized  in 
a  shallow  dish;" 

4.  Measuring   the   total   volume   of   silver   bromide   in   a 
photographic  emulsion,  determining  the  number  of  grains  in 
one  cc.  of  the  emulsion,  and  calculating  the  mean  diameter 
of  the  grains  gives  the  data  for  computing  the  average  thick- 
ness of  the  silver  bromide  crystals  in  the  emulsion.     To  do 
this,  a  given  quantity  of  emulsion  was  spread  on  a  definite 
area   of   film.     Pieces   were   then   taken   from   five   different 
regions  of  the  film  and  by  means  of  a  microtome  three  cross 
sections  of  each  of  5^  (dr  0.06)  in  thickness,  were  prepared 
and  used  for  the  microscope  preparations.     This  process  was 
repeated  for  five  different  emulsions,  each  containing  a  dif- 

1  Koch,  P.  P.,  and  du  Prel,  G.,  Ueber  das  Korn  der  photographischen  Platte  und  eine 
Methode  zu  einer  Untersuchung.     Physik.  Zeits.  17:  536.  1916. 

2  Krohn,  F.  W.  T.t  1.  c. 

93 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

ferent  quantity  of  silver  bromide,  so  that  there  were  75  prep- 
arations in  all.  One  hundred  and  fifty  photomicrographs 
(magnification  1176.4  diameters)  were  taken,  using  a  para- 
boloid condenser.  The  use  of  dark  field  illumination  makes 
it  possible  to  define  each  grain  sharply,  so  that  the  number 
of  grains  in  the  emulsion  can  be  counted.  The  number  in 
these  emulsions  varied  from  16  to  43  x  1010  per  cc.  One 
emulsion  contained  16  x  1010  dr  0.  5  x  10f>  grains.  The  diameter 
of  the  grains  averages  about  1 . 5/^.  Assuming  that  all  the 
grains  are  truncated  tetrahedra,  the  average  thickness  may 
be  reckoned  as  1/14  of  the  diameter.  In  other  words,  most 
of  the  grains  in  photographic  emulsions  are  plates. 

The  dark  grains.  As  shown  in  Fig.  22,  these  grains  show 
very  intense  light  between  crossed  nicols.  This  may  be  due 
either  to  a  thicker  layer  of  silver  iodo-bromide  or  to  a  greater 
reflection  of  light.  Also,  there  is  a  layer-by-layer  variation 
in  the  definition  of  these  dark  grains  as  the  focus  of  the  micro- 
scope is  changed.  All  this  indicates  that  these  dark  grains 
are  crystals  more  or  less  well  developed  in  all  three  dimensions, 
and  that  the  effect  of  dark  color  is  probably  produced  by  the 
strong  light-reflections  in  the  crystal,  due  to  the  unusually 
high  refractive  index,  as  has  been  observed  in  crystals  of 
thallium  salts  and  of  gold-barium  acetate. 

The  pentagonal  dodecahedra  will  be  discussed  later.  (See 
page  95.) 

FACES    OBSERVED 

The  following  faces  were  observed  in  the  different  crystal 
forms  : 

Usual  crystals \  1 1 1  \ 

Combinations <{lllf>      + 

UH}-    + 

Plates \\\\\ 

Combinations "{Ill        -(-      \  100 

+     ^210 
UOO 


an 


Needles ml 

Combinations \\\\\      +     j  100 

Etching. 

In  order  to  study  the  etch-figures  which  appeared  on  the 
larger  octahedral  faces  after  treatment  with  ammonia,  the 
crystalline  precipitate  obtained  by  cooling  the  ammoniacal 
solution  was  kept  in  the  closed  bottle  at  room  temperature 
without  removing  the  ammonia.  The  solubility  increased 
with  the  higher  temperature  and  very  beautiful  etch-figures, 
in  the  form  of  triangles  with  rounded  corners,  resulted.  It 
was  thought  that  these  figures  would  aid  in  determining 
symmetry-ratios,  but  in  this  case  they  gave  no  criterion  for 
classification. 

94 


CHAPTER  VII 


THE    CLASSIFICATION    OF    SILVER   BROMIDE    CRYSTALS 

From  the  crystal  forms  observed,  it  would  seem  that  the 
symmetry-relations  of  silver  bromide  crystals  are  less  than 
is  now  assumed.  An  accurate  method  of  investigating  this 
relation  by  the  determination  of  different  physical  constants 
in  various  directions  in  the  silver  bromide  crystals,  which 
would  be  adapted  to  the  extremely  small  dimensions  of  the 
silver  bromide  crystals,  is  not  yet  perfected. 

The  pentagonal  dodecahedra  obtained  by  cooling  the 
ammoniacal  solution  are  so  important  for  the  classification 

of  silver  bromide  that  they  merit 
closer  study.  These  forms  oc- 
curred only  in  combination  with 
the  octahedra,  as  shown  in  Figs. 
34  and  35.  This  combination  is 
well  known  in  SnI4,  FeS2,  CoAsS, 
Cs2Al2.(SO4)4.24H2O  and  (NH- 
(CH3)3)2.A12(SO4)4.24H2O.  Un- 
fortunately, the  combinations  of 
pentagonal  dodecahedra  and  oc- 
tahedra are  not  very  clear  in  the 
accompanying  reproductions,  in 
which  the  combinations  resemble 
quintettes.  But  a  careful  study 
of  the  original  preparations  leaves 
no  doubt  that  these  crystals, 
especially  those  in  Fig.  35  d  and 
f,  are  combinations  of  true  pentagonal  dodecahedra  with 
octahedra. 

The  pentagonal  dodecahedra  are  distinguished  from  the 
usual  silver  bromide  crystals  in  that  there  is  a  strong  tendency 
to  plate-formation,  while  the  faces  ABF,  BGF,  BCG,  CHG, 
CDH,  DJH,  DEJ,  EKJ,  EAK  and  AFK  show  a  much  greater 
rapidity  of  growth  in  the  direction  of  their  normal  than  the 
other  faces.  The  resulting  lack  of  development  of  these  latter 
faces  produces  a  five-sided  pyramid  which  in  Fig.  35a  is  lying 
with  one  of  the  triangular  faces  on  the  slide. 

In  Fig.  35a  and  b  are  pictured  two  combinations  {ill}  "i" 
[210}  in  which  the  upper  and  lower  pyramids  are  about  the 
same  size.  Since,  as  Fig.  34  shows,  the  corners  of  the  two 
pyramids  do  not  coincide,  but  are  about  36°  apart  and  are 

95 


FIG.  34 

Diagram  showing  fully  de- 
veloped combination  of  octa- 
hedron and  pentagonal  dode- 
cahedron-. 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 


FIG.  35 

Combinations  of  octahedra  with  pentagonal  dodecahedra  of  silver 
bromide.     Enlarged  800  diameters 

96 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

flattened,    the   resulting   truncation    produces   a   decagon    in 
projection. 

The  occurrence  of  pentagonal  dodecahedra  shows  that 
silver  bromide  crystals  belong  only  to  the  tetrahedral- 
pentagonal-dodecahedral  or  the  dyakisdodecahedral  class.1 
However,  there  is  no  evidence  of  the  hemihedrism  of  silver 
bromide  crystals.  What  have  been  described  by  many 
authors  as  tetrahedra  of  silver  bromide  in  photographic 
emulsions  are  triangular  octahedral  plates  (see  below).  Inas- 
much, therefore,  as  this  hemihedrism  is  not  proven,  we  must 
place  silver  bromide  in  the  dyakisdodecahedral  class. 

THE   CLASSIFICATION   OF    SILVER   CHLORIDE 
AND   SILVER   IODIDE 

It  has  been  mentioned  that  silver  iodide  in  limited  quanti- 
ties can  form  homogeneous  mixed  crystals  with  silver  bromide. 
This  indicates  that  the  metastable  silver  iodide,  crystallized 
in  regular  form  from  solutions  at  ordinary  temperatures,  may 
be  placed  in  the  same  class  as  silver  bromide. 

Groth2  placed  silver  chloride  in  the  hexakisoctahedral 
class,  in  accordance  with  his  observations  of  the  faces  of 
natural  crystals.  But  these  crystals  may  be  as  logically 
placed  in  the  dyakisdodecahedral  class.  Furthermore,  Thiel3 
has  demonstrated,  by  the  continuous  change  of  potential  of 
silver  chloride-silver  bromide  mixtures,  that  silver  chloride 
forms  homogeneous  mixtures  with  silver  bromide  in  all  pro- 
portions. So  in  all  probability  silver  chloride  belongs  to  the 
same  crystal  class  as  silver  bromide.4 

THE    POSSIBILITY    OF    MODIFICATIONS    OF    SILVER    BROMIDE 

Even  though  the  reasons  for  assuming  the  existence  at 
ordinary  temperatures  of  a  stable  or  metastable  hexagonal 
silver  bromide  in  addition  to  the  regular  silver  bromide  are 
insufficient,  the  fact  that  silver  bromide  crystallizes  in  the 
dyakisdodecahedral  class  indicates  that  it  is  possible  for  it  to 
crystallize  in  right  and  left  pentagonal  dodecahedra,  so  that  a 
kind  of  enantiomorphism,  as  in  quartz,  sodium  chlorate,  etc., 
may  exist.  This  is  not  to  be  interpreted  in  a  purely  geomet- 
rical sense,  i.  e.,  that  optical  differences,  such  as  the  opposite 

1  See  concordance,  p.  131. 

2  Groth,  P.,  1.  c.,  Vol.  I.,  p.  200. 

3  Thiel,  A.,  1.  c. 

4  Crystals  were  also  identified  from  silver  cyanide  and  silver  sulphocyanide  as  octahedra 
of  the  regular  system.     These  investigations,  however,  have  not  been  carried  further. 

97 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

rotation  of  circularly  polarized  crystals,  can  occur  here,  but 
rather,  as  Marbach1  found  in  the  case  of  pyrite  and  cobaltite, 
that  these  crystals  belong  in  the  thermo-electric  electromotive 
series  partly  beyond  positive  antimony  and  partly  beyond 
negative  bismuth:  so  that  a  pyrite  crystal  of  the  first  kind 
which  is  united  with  a  similar  crystal  of  the  second  kind 
produces  a  stronger  thermal  current  than  antimony  with 
bismuth.  In  this  sense  it  is  possible  for  silver  bromide  to 
form  two  modifications. 

1   Marbach,  cited  by  Groth  in  Physikalische  Krystallographie,  p.  193.    (Third  Edition.) 


98 


CHAPTER  VIII 

The  Silver  Bromide  Crystals  of 
Photographic  Emulsions 

As  is  well  known,  in  photographic  emulsions  the  silver 
bromide  is  precipitated  in  the  presence  of  a  protective  colloid 
(gelatine)  and  therefore  one  has  to  do  with  a  case  of  colloidal 
precipitation.  Since  there  are  distinct  silver  bromide  crystals 
in  the  melted  emulsion  and  in  plates,  a  transformation  in  the 
sense  of  colloidal  silver  bromide  to  crystalline  silver  bromide 
must  have  taken  place. 

This  transformation  is,  however,  simply  a  special  case 
of  the  thermodynamic  principle  according  to  which  the  system 
tends  to  reduce  its  surface-energy,1  and  which  (as  has  been 
indicated  in  Chapter  V),  is  the  starting  point  of  the  Gibbs- 
Curie-Wulff  law;  for,  as  stated,  this  conversion  takes  place 
more  quickly  in  the  presence  of  a  solvent  of  silver  bromide, 
such  as  potassium  bromide,  ammonium  hydroxide,  etc. 
Further,  the  rapidity  of  the  transformation  increases  with 
rise  in  temperature.  Whether  the  colloidal  silver  bromide 
consists  of  an  aggregation  of  unusually  small  crystals,  or  of 
purely  amorphous  silver  bromide — i.  e.,  in  the  molecular 
state — makes  no  difference,  since  the  surface-energy  of  colloids 
is  very  large  in  proportion  to  the  surface-energy  of  the  single 
silver  bromide  crystals.  It  is  probable  therefore  that  the 
velocity  of  solution  of  colloidal  silver  bromide  is  greater 
that  that  of  the  crystalline  silver  bromide.  Thus  a  trans- 
formation occurs  in  the  solvent  by  which  the  crystalline 
silver  bromide  is  formed  while  the  colloidal  form  disappears. 
This  is  essentially  the  same  as  the  principle  discussed  above 
to  indicate  the  relation  between  the  size  of  the  crystal  and 
the  decreased  solubility,  which  in  photographic  literature  is 
known  as  "Ostwald  Ripening."2 

Therefore  in  photographic  emulsions  one  finds,  certainly 
for  a  number  of  the  grains  if  not  for  all,  the  same  conditions 
as  in  ordinary  processes  of  crystallization;  and  this  does  not 
prevent  our  applying  the  Gibbs-Curie-Wulff  law  to  at  least 
the  larger  silver  bromide  grains  which  have  definite  crystalline 
structure. 

1  In  general,  small  crystals  dissolve  more  quickly  than  larger  ones  because,  in  the 
transformation  of  many  smaller  crystals  into  one  larger,  the  surface  energy  is  diminished. 

2  Ostwald,  W.,  1.  c. 

99 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

The  question  of  the  influence  of  gelatine  on  crystallization 
processes,  already  discussed  in  part,  will  be  dealt  with  in 
another  relation  in  a  later  chapter. 

FORMS    OF    SILVER   BROMIDE   CRYSTALS   IN   EMULSIONS 

The  forms  of  the  silver  bromide  crystals  in  photographic 
emulsions  are  very  varied,  though  all  belong  to  the  same 
crystallographic  system  and  the  same  class.  In  122  different 
emulsions  which  were  examined  at  a  magnification  of  2,500 
diameters,  only  octahedra  could  be  positively  identified. 
Higson  (1.  c.)  says  he  found  cubic  crystals  in  silver  bromide 
emulsions.  In  the  work  described  here  this  observation  could 
not  be  confirmed,  and  the  presence  of  cubes  and  of  combina- 
tions of  cubes  with  octahedra  or  other  forms  could  never  be 
definitely  determined.  It  is  possible  that  Higson's  cubic 
crystals  were  obtained  from  precipitations  from  dilute  am- 
moniacal  solutions. 

The  octahedra  appear: 

a.  In  crystals  more  or  less  well  developed  in  three  directions 
— dark  grains; 

b.  In  plates,  which  are  most  markedly  developed  in  two 
directions;  and 

c.  In    needles,    which    are    developed    principally    in    one 
dimension. 

(Those  grain-aggregations  and  groups  which  come  only 
within  the  range  of  probability  will  be  disregarded.) 

Each  of  these  forms  exhibited  variations,  for  which  the 
plates  are  especially  noticeable,  though  the  needles  and 
ordinary  crystals  showed  similar  differences  to  a  less  marked 
degree.  We  will,  therefore,  limit  our  discussion  here  to  the 
plates,  since  they  are  in  the  majority  in  photographic 
emulsions. 

Plate-forms  are  so  numerous  that  it  is  impossible  to  describe 
them  all.  Here  we  shall  discuss  only  definite  types  as  observed 
in  the  emulsions,  remembering  that  the  other  forms  represent 
all  possible  transitions  between  the  types  described,  and  that 
all  are  variations  of  one  and  essentially  the  same  crystal  form. 

Fig.  36  represents  an  octahedron  (ABCDEF)  which  is 
lying  with  one  face  on  the  paper.  In  parallel  projection  a 
hexagon  is  obtained.  Now,  if  the  capillary  constant  between 
the  mother  liquor  and  the  crystal  is  different  for  different 
faces,1  so  that,  for  example,  Kj  of  the  face  AED  and  K2  of 

1  Cf.  Chapter  V. 

100 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS^ 


the  face  BCF  are  much  smaller  than  K3  of  the  face  ACD, 
K4  of  the  face  CDF,  K5  of  the  face  DFE,  K6  of  the  face  FEE, 
K7  of  the  face  EBA,  and  K8  of  the  face  BAG,  then  the  condi- 
tions necessary  for  the  formation  of  a  plate  or  tablet  are 
established.2 

An  equilateral  regular  hexagon  is  produced  if 

Ki  =   K2,  K3  -   K4  -   K5  =   Ke  =   K7  =   K8, 

K,  <  K8. 

These  plates  are  thinner  the  greater  the  ratio  K3/Ki.  For 
one  emulsion  the  mean  value  of  K3/Ki  has  been  determined 
as  14. 

On  the  other  hand,  if 

1C  =    K2,  K3  =    K5  -    K7,  K4  -    K6   =    K8,  and 

K!  <  K3  <  K4  <  2K3, 

a  scalene  but  otherwise  regular  hexagon  will  result,  as  shown 
in  Fig.  37.  Here  AED  is  the  upper  and  BCF  the  lower 
octahedral  face  of  the  tablet. 


FIG.  36 

Diagram  showing  the 
formation  of  a  tabloid 
equilateral  hexagon  from 
an  octahedron. 


FIG.  37 

Diagram  showing  the 
formation  of  a  tabloid  un- 
equilateral  hexagon  from 
an  octahedron. 


If  the  following  conditions  obtain  during  development, 
K4  ^  2K3,  a  triangular  plate  will  be  formed,  as  shown 
in  Fig.  38.  Since  the  growth  of  th.-ee  of  the  side-faces  is 
two  or  more  times  as  rapid  as  that  of  the  other  three,  the 
latter  are  suppressed. 

Another  scalene  but  otherwise  regular  hexagon  which  is 
often  to  be  seen  in  emulsions  is  formed  when 

K!  -    K2,  K3  =    K4  =    K6   =    K7>  K6  =    K8,  and 
K!  <  K3  <  K5  <  2K3. 

2|For  the  sake  of  simplicity,  Ki  is*assumed  as  equal  to  K2.     Small  differences  between 
Ci  and  K2  obviously  do  not  affect  this  and  the  following  results.      It  is  also  assumed  that 


Ki  and  K2  obviously 

K  does  not  change  during  growth. 


101 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 


Irregular  forms  appear  when  K3,  K4,  K5,  KG,  K7,  and  K8 
are  more  or  less  unequal. 

One  very  remarkable  form  occurring  in  emulsions  is  the 
trapezoidal  lamina  (Fig.  39)  which  appears  in  the  most  varied 
modifications.     The  conditions  for  its  formation  are: 
Kx  =   K2,  K3  -   K5  =   K7  =K8,  K4  -   K6, 
K!  <  K3  and  K4  ^  2K3. 


F 

FIG.  38 

Diagram  showing  the  forma- 
tion of  a  tabloid  equilateral  tri- 
angle from  an  octahedron. 


Diagram  showing  the  forma- 
tionof  a  tabloid  trapezium 
from  an  octahedron. 


Modifications    result    when : 

K5  =  K8,  K5  <  K4;  K8  <  K4,  K3  =  K7  <  K4,  or  if 
under  the  same  conditions: 

K3  -    K7,  K3  <  K4,    K7  <  K4,  K5  -    K8  <  K4,  etc. 

A  pentagon  (Fig.  40)  with  angles  of  60°  and  120°  and 
therefore  of  a  form  quite  different  from  the  faces  of  the  pentag- 
onal dodecahedron  appears  when: 

IVl      =       X\-2»     K-3      ==       K-4      ==       KG       —       .K--     =       K-8) 

K:  <  K3  and  K5   ^    2K3. 

Modifications  are  obtained  when  K3,  K4,  K6,  K7,  and  K8 
are  more  or  less  unequal,  but  always  remain  smaller  than 
K5. 

A  rhombus  (Fig.  41)  with  angles  of  60°  and  120°  appears 
when  two  parallel  side  faces  are  suppressed  on  account  of  too 
great  velocity  of  development,  when  therefore: 

K!  =   K2,  K3  =   K4  -   K6   -   K7,  K6  =   K8> 

K,  <  K3  and  K6   ^   2K3. 

102 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

Modifications  may  appear  when  K3,  K4,  K6,  and  K7  are  more 
or  less  varied,  but  always  remain  smaller  than  K5. 


FIG.  40 

Diagram  showing  the  forma- 
tion of  a  tabloid  pentagon  from 
an  octahedron. 


FIG.  41 

Diagram  showing  the  forma- 
tion of  a  tabloid  rhombus  from 
an  octahedron. 


A  different  development  phenomenon  is  responsible  for 
the  needle  formations,  which  develop  when  the  free  energy 
of  the  base  is  greater  than  that  of  the  prism-faces,  so  that  the 
crystal  grows  most  rapidly  in  the  direction  of  the  base.  The 
multiplicity  of  plate-forms  is  induced  by  the  greater  velocity 

of  growth  of  one  side-face  between 
two  other  side-faces  having  an  inferior 
development.  The  formation  of  plate- 
shaped  crystals  by  convection-cur- 
rents in  the  mother  liquor  is  not  pos- 
sible in  silver  bromide  emulsions,  since 
the  grains  are  suspended  and  in  slight 
Brownian  movement,  which  reduces 
the  convection-currents,  on  the  one 
hand,  and  these  are  further  reduced 
by  the  viscosity  of  the  mother  liquor 
on  the  other.  An  octahedral  needle 
results  when  two  contiguous  side-faces 
develop  more  rapidly  than  the  other 
side-faces.  (Fig.  42.)  However  rapid 
this  growth  may  be,  these  latter  faces 
can  never  disappear.  But  if  the  capil- 
larity constant  of  one  of  these  rapidly 
growing  faces  decreases  during  growth, 
the  development  of  the  entire  needle  in  one  direction  is 
interfered  with. 


F 

FIG.  42 

Diagram  showing  the 
formation  of  a  needle  from 
an  octahedron. 


103 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

Among  needles,  one  must  differentiate  between  well- 
developed  needles  of  approximately  equal  thickness  and 
width,  and  plate-like  needles.  The  first  type  appears  as 
octahedra  when  Ki  =  K2  =  K5  =  K8.  The  second  appears 
as  octahedra  when  Ki  <  K5.  These  distinctions  could  not  be 
made  in  photographic  emulsions,  however,  because  of  the 
limits  of  microscopic  resolving  power. 

The  conditions  for  the  formation  of  a  needle  are: 
Id  =  K2,  K3  -  K4  -  K6  =  K7,  K5  =  K8,  and  K3  <  K5. 

Now  it  is  possible  that  needles  do  not  develop  with  equal 
rapidity  in  two  opposite  directions — e.  g.,  K3  and  K4  may 
be  very  much  greater  than  KG  and  K7.  But  this  does  not 
affect  the  results  in  the  least. 

Only  slight  indications  of  tabular  octahedral  twinning, 
such  as  was  frequently  observed  in  ammoniacal  silver  bromide 
crystal-formation  (Fig.  43),  were  to  be  seen  in  completed 
photographic  emulsions.1 

THE  RELATION  BETWEEN  THE  LIGHT-SENSITIVITY  AND  THE 

SURFACE-ENERGY   OF    SILVER   BROMIDE   CRYSTALS 

IN   EMULSIONS 

Only  very  general  statements  can  be  made  on  this  subject, 
as  very  little  is  known  about  it.  If  a  change  in  light-sensi- 
tivity is  produced  by  an  increase  or  decrease  of  the  surface- 
energy  of  silver  bromide  emulsion  crystals,  then  in  general 
it  may  be  assumed  that  larger  emulsion  grains  have  a  different 
sensitivity  from  smaller.  In  practice,  indeed,  it  often  seems 
that  the  coarse-grained  emulsions  are  more  sensitive  than 
the  fine-grained.  But  that  this  is  not  always  the  case  is 
shown  by  the  following: 

An  experimental  emulsion  was  prepared,  the  grains  of 
which  measured  up  to  S/JL  in  diameter  and  which  had  an  H. 
and  D.  speed  of  only  38.  In  comparison  with  this  emulsion 
a  "Royal  Standard  Lightning  Plate"  from  Kodak  Ltd.  was 
tested,  the  grains  of  which  averaged  up  to  2.8^  in  diameter, 
and  of  which  the  H.  and  D.  speed  was  728.  Thus  it  appears 
that  emulsions  containing  grains  of  approximately  one-third 
the  linear  dimensions  are  more  than  nineteen  times  as  sensitive. 
This  is  true  also  of  individual  grains  in  the  same  emulsion. 
After  a  quantitative  investigation  of  this  question,  Koch  and 
du  Prel2  concluded  that  it  is  not  possible  to  formulate  a  definite 
relation  between  grain-size  and  sensitivity  with  the  information 
at  present  available,  but  that  it  is  certain  that  the  largest 
grains  in  an  emulsion  are  by  no  means  the  most  sensitive. 

1  See  Chapter  V,  p.  68. 

2  Koch,  P.  P.,  and  du  Prel,  G.,  1.  c. 

104 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 


FIG.  43 

Twin  forms  on  octahedral  faces  of  silver  bromide.     A-E,  magnified 
800x;    F,  2500x 

105 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

Lehmann  and  Knoche1  also  demonstrated  that  ripening 
increases  the  sensitivity  of  silver  bromide-albumen  emulsions 
without  noticeably  altering  the  size  of  the  grain. 

Now,  if  one  considers  the  ripening  process  as  a  purely 
thermodynamic  one,  entirely  independent  of  light-sensitivity,2 
(i.  e.,  as  a  process  in  which  the  surface-energy  of  the  grains 
tends  to  reach  a  minimum),  Liippo-Cramer's  remarks3  to 
the  effect  that  the  sensitivity  of  colloid  emulsions  may  be 
increased  to  but  a  relatively  slight  degree,  and  that  the  initial 
stage  of  emulsion-making  (the  conditions  under  which  the 
silver  bromide  is  precipitated),  is  of  much  greater  importance 
than  the  later  steps  of  the  process,  become  more  intelligible. 

In  other  words,  the  increase  in  sensitivity  is  determined 
not  so  much  by  the  ripening  process  as  by  the  conditions 
under  which  the  silver  bromide  precipitation  was  accomplished. 
(See  particularly  Chapter  II,  p.  27.) 

Luther's  statement,  made  independently,  that  the  laws  of 
thermodynamic  equilibrium  are  not  applicable  to  photo- 
chemical equilibrium,  and  that  therefore  there  is  no  definite 
connection  between  thermodynamics  and  photochemistry,4 
is  in  entire  agreement  with  this  view. 

Thus  we  come  to  the  conclusion  that  the  high  sensitivity 
of  some  photographic  emulsions  may  be  somewhat  influenced 
by  variations  in  the  surface-energy,  but  can  certainly  not  be 
entirely  determined  thereby.  In  agreement  with  Liippo- 
Cramer's  statement  above,  we  must  very  probably  seek  these 
conditions  in  the  crystal  structure  of  the  silver  bromide  on 
the  surface  of  the  grains.5 

From  the  crystallographic  standpoint  only  one  method  of 
investigation  in  this  direction  is  possible  without  methods  of 
X-ray  crystal  analysis.  That  method  is  to  determine  the 
directions  of  most  rapid  growth  relative  to  the  characteristics 
of  the  crystal  lattice,  an  observation  which  is  especially 
significant  in  the  case  of  the  silver  bromide  octahedra.  And 
since  these  octahedra  occur  in  three  forms, — as  ordinary 
crystals,  plates,  and  needles — it  is  possible  to  investigate  the 
directions  of  greatest  growth  not  only  in  volume,  but  even  on 
the  octahedral  faces. 

1  Lehmann,  E.,  and  Knoche,  P.,  Plate-grain  and  albumen  emulsions.  B.  J.  Phot.  61: 
759.  1914. 

2  The  H.  and  D.  interpretation  of  light  sensitivity  is  intended  here. 

3  Luppo-Cramer,  Phot.  Prob.,  p.  35. 

4  The  formation  of  a  thermodynamically  more  stable  form  of  silver  bromide  does  not 
indicate  that  such  a  form  is  less  sensitive  to  light,  as  R.  Abegg  (Die  Silberkeimtheone  des 
latenten  Bildes.     Arch.  wiss.  Phot.  1:  18.  1899)  and  V.  Bellach  (1.  c.,  p.  37)  assume. 

s  W.  D.  Bancroft  (The  photographic  plate,  1.  c.)  and  W.  Reinders  (1.  c.)  attribute  the 
high  sensitivity  to  the  presence  of  gelatine.  But  it  must  be  borne  in  mind  that  emulsions 
of  unusually  varied  sensitivities  may  be  prepared  from  the  same  gelatine — which  may  be 
explained  by  differences  in  structure. 

106 


CHAPTER  IX 

THE    DIRECTIONS    OF    MOST    RAPID     GROWTH    IN    SILVER    BROMIDE 

CRYSTALS    AND    THE    OCCURRENCE    OF 

ANOMALOUS    FORMS 

1.  In   the    ordinary    octahedra.     According    to    Lehmann,1 
the  directions  of   most  rapid   growth   are   along   the  line  of 
greatest   acumination.     Since   in   octahedra   these   directions 
coincide  with  the  three  principal  crystallographic  axes  of  the 
crystal,   the  former  are  indicated  by  the  latter.     Lehmann 
has  also  shown  that  skeletons  grow  in  these  directions,  and 
silver  bromide  skeletons  which  were  formed  exactly  like  the 
framework    of    the    three    co-ordinate    axes   were    repeatedly 
found. 

2.  In   octahedral   plates.     To  determine  the  direction     of 
most  rapid  development  here,  either  of  two  methods  may  be 
followed : 

a.  By    observing    the    direction    in    which    the    skeleton 
develops ; 

b.  By  very  rapid  but  suddenly  disturbed  crystallization. 
In    addition    to    the   skeletons   which    coincide   with    the 

crystal  axes,  there  is  in  plates  one  form  of  skeleton  the  rays 
of  which  coincide  with  the  edges  of  vicinal  faces,  and  which 
may  be  termed  surface  skeletons.2  They  must  not  be  regarded 
as  special  needle-combinations,  for  they  always  appear  in 
uniform  and  entirely  regular  star-shaped  figures  from  which 
three  definite  rays  emanate  at  angles  of  usually  120°.  These 
skeletons  often  form  the  framework  of  plates,  when  they  have 
three,  six,  nine,  or  twelve  rays.  Four  rays  usually  appear 
only  in  trapezoidal  plates,  and  five  in  octahedral  pentagonal 
plates.3 

Structural  anomalies  occurred  so  frequently  that  they 
merit  special  attention.  Crystal-development  is  possible 
only  when  the  mother-liquor  is  slightly  supersaturated.  The 
surplus  quantity  of  the  crystalline  substance  in  solution  is 
deposited  and  thus  the  concentration  of  that  part  of  the 
liquid  in  the  vicinity  of  a  crystal  is  decreased.  By  the  liber- 
ation of  the  latent  heat  of  solution  and  the  decrease  in  satur- 

1  Lehman,  O.,  cited  by  Groth  in  Physikalische  Krystallographie,  p.  284. 

2  The  ice-skeletons  of  snow  flakes  may  be  classified  here. 

3  These  trapezoidal  rays  can  be  crystallographically  constructed  from  a  vicinal  triakis- 
octahedral  plate  (triangle)  in  which  one  of  the  edges  between  two  vicinal  faces  is  truncated 
by  a  yicinal-ikositetrahedron.     Similarly,  the  five  rays  of  the  octahedral  pentagon  may  be 
explained  by  the  appearance  of  two  vicinal  ikositetrahedra.     The  octahedral  rhomboids 
occur  so  seldom  that  their  rays  have  not  been  satisfactorily  determined. 

107 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

ation,    the    solution    becomes    specifically    lighter    and    local 
concentration  and  convection  currents  set  in. 

According  to  Lehmann,1  these  local  concentration-currents 
are  the  cause  of  the  formation  of  structural  anomalies  and 
skeletons,  and  Wulff2  considers  them  the  cause  of  the  formation 
of  vicinal  faces.3 

Among  the  plates  obtained  from  ammoniacal  solution 
were  a  large  number  of  structural  anomalies,  a  few  of  which 
are  illustrated  in  Figs.  44  and  45.  That  such  lamellar- 
structures  actually  are  produced  may  be  demonstrated  by 
the  variations  in  the  Newton  colors.  That  vicinal  faces 
having  absolutely  no  connection  with  the  crystal  faces  may 
be  simulated  in  this  way  is  shown,  e.  g.,  in  Fig.  44a,  where 
there  are  indications  of  an  unusually  flat  ikosi tetrahedron, 
while  the  other  plates  and  those  in  Fig.  45  show  traces  of 
unusually  flat  triakisoctahedra.  Fig.  46  shows  the  same 
much  more  sharply  defined,  and  Fig.  46d  is  especially  remark- 
able for  the  combination  of  a  triakisoctahedral  skeleton4 
with  an  octahedral  plate.  Thus  one  has  here  a  transition  to 
the  skeleton.  That  these  skeletons  may  appear  isolated  is 
shown  in  Fig.  47,  where  twelve  triakisoctahedral  silver  bromide 
skeletons  are  reproduced.  In  addition  to  these,  a  large 
number  of  ikositetrahedral  skeletons  of  silver  bromide  were 
found,  twelve  of  which  are  pictured  in  Fig.  48.  Ikositetrahedral 
skeletons  in  octahedral  plates  are  shown5  in  Fig.  49,  and 
combinations  of  triakisoctahedral  and  ikositetrahedral  skele- 
tons in  octahedral  plates  in  Fig.  50,  among  which  Fig.  50f 
is  noteworthy  because  of  the  indications  of  a  right  and  left 
dyakisdodecahedral  skeleton  in  combination  with  the  triakis- 
octahedral and  the  ikositetrahedral  skeleton  in  an  octahedral 
plate. 

The  attempted  classification  of  skeletons  does  not  pretend 
to  be  conclusive,  because  of  the  difficulties  of  establishing  an 

1  Lehmann,  O.,  Molekular  Physik.  Vol.  I.,  p.  354;  and  Groth,  P.,  1.  c.,  p.  284. 

2  Wulff,  J.,  1.  c. 

3  H.  A.  Miers  (An  enquiry  into  the  variations  of  angles  observed  in  crystals.     Phil. 
Trans.  A.  202:  459.  1903),  attempted  to  destroy  the  concentration-currents  by  stirring  the 
liquid  (a  solution  of  alum)  but  even  so  obtained  vicinal  faces.     Then  he  demonstrated  by 
refractometric  measurements  that  the  refractive  indices  of  the  solution  which  is  in  contact 
with  the  crystals  is  the  same  as  that  of  a  strongly  supersaturated  solution.     This  adsorption- 
layer  is,  of  course,  quite  different  from  the  solution  itself,  so  the  real  nature  of  the  vicinal 
faces  is  still  not  entirely  clear. 

4  So  far  as  we  know,  crystallographers  have  not  introduced  any  especial  differentiations 
nor  a  nomenclature  for  skeletons.     The  silver  bromide  skeletons  are  here  designated  ac- 
cording to  the  theoretical  vicinal  faces  of  which  they  may  form  the  edges,  without,  however, 
considering  them  capable  of  being  classified  crystallographically. 

5  Ikositetrahedral  skeletons  in  octahedral  laminae  are  very  seldom  obtained  in  the 
above-described  methods  of  precipitation.     Those  reproduced  here  were  obtained  by  add- 
ing a  mixture  of  gum  arabic  and  dextrose  in  varying  quantities  to  the  solution,  and  heating 
it  to  not  higher  than  60°  C. 

108 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 


FlG.  44 

Lamellae  formations  on  octahedral  faces  of  silver  bromide. 
Magnification,  800  diameters. 


109 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 


FlG.  45 

Lamellae  formation  on  octahedral  faces  of  silver  bromide. 
Magnification,  800  diameters 

110 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 


FlG.  46 

Ikositetrahedral   skeletons   on   octahedral   faces   of   silver  bromide 
crystals.     Magnified  800  times 


111 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 


FlG.  47 
Triakisoctahedral  skeletons  of  silver  bromide,  enlarged  800  diameters 


112 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 


FIG.  48 
Ikositetrahedral  skeletons  of  silver  bromide,  enlarged  800  diameters 


113 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 


No.  49 

Ikositetrahedral  skeletons  on  octahedral  faces  of  silver  bromide 
crystals.     Magnified  800  times 

114 


FIG.  50 

A-E,  Combinations  of  triakisoctahedral  and  ikositetrahedral  skele- 
tons on  octahedral  faces  of  silver  bromide,  magnified  800  times. 

F,  Diagrammatic  combination  of  triakisoctahedral,  ikositetrahe- 
dral, r-  and  1-  dyakisdodecahedral  skeleton  on  an  octahedral  face  of 
a  silver  bromide  crystal.  Magnification,  800  diameters. 

115 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

exact  classification;  for  instance,  the  differences  between 
triakisoctahedral  and  ikositetrahedral  skeletons  are  not 
always  evident.  The  ends  of  the  rays  vary,  being  either 
pointed  or  blunt.  Skeletons  having  pointed  rays  have  been 
attributed  to  the  triakisoctahedra,  those  having  blunt  rays 
to  the  ikositetrahedra.  To  which  of  these  groups  Fig.  47c 
belongs  is  not  clear;  but,  since  the  writer  was  able  to  observe 
this  skeleton  as  a  plate  with  an  octahedral  face,  it  has  been 
assigned  to  the  triakisoctahedra. 

The  connection  between  skeleton  forms  and  structural 
anomalies  due  to  lamination  is  thus  very  evident  in  the  case 
of  silver  bromide.  There  is,  therefore,  no  objection  to  Leh- 
mann's  theory  that  these  forms  are  the  results  of  special 
growth  conditions  caused  by  local  concentration  currents. 
But  it  must  be  remembered  that  under  these  special  growth- 
conditions  regularities  appear  which  can  be  conditioned  only 
by  the  lattice  structure  of  the  crystals,  as,  for  example,  the 
directions  of  the  rays  of  the  skeleton.  Then  these  rays 
probably  represent  the  directions  of  most  rapid  development. 
The  diagram  in  Fig.  50f  contains  all  the  skeletal  radiations 
thus  far  verified;  from  which  we  conclude  that  in  the  octa- 
hedral face  there  are  twelve  possible  directions  of  most  rapid 
growth,  which  form  angles  of  30°  with  one  another. 

The  second  method  for  determining  the  direction  of  most 
rapid  growth,  by  the  sudden  interruption  of  unusually  rapid 
crystallization  processes,  consists  in  greatly  diluting  the 
ammoniacal  silver  bromide  solution  with  water.  (See  table 
on  page  86.)  The  crystals  obtained  in  this  way  show  growth 
phenomena  at  the  edges  of  the  octahedral  plates,  as  indicated 
in  Fig.  51.  The  directions  of  growth  of  the  triakisoctahedral 
and  ikositetrahedral  skeletons  of  the  octahedral  plates  are 
represented  in  the  center  of  gravity  of  the  triangle,  but  for  the 
sake  of  clarity  the  development  directions  of  both  dyakis- 
dodecahedral  skeletons  are  omitted.  If  one  transposes  the 
directions  of  growth  at  the  corners  so  that  they  start  from 
the  center  of  gravity,  it  is  obvious  that  they  coincide  exactly 
with  those  of  all  skeletal  forms. 

3.  In  octahedral  needles.  In  needles  the  direction  of  most 
rapid  growth  obviously  coincides  with  the  needle  axis.  In 
ordinary  needles  this  direction  coincides  with  one  of  the 
principal  crystallographic  axes.  If  a,  b,  and  c  are  the  three 
main  axes,  having  development-velocities  of  Va,  Vt>,  and  Vc, 
then  to  form  a  needle  only  one  of  the  three  need  have  a  much 
greater  velocity  of  growth  than  the  other  two,  i.  e.,  Va  =  Vt>, 
Vb<Vc. 

116 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

The  directions  of  most  rapid  growth  in  laminate  needles 
(see  Fig.  42)  may  be  determined  from  the  octahedral  hexagon, 
since  these  needles  are  distorted  hexagons.  As  is  readily 
seen,  this  direction  coincides  with  one  of  the  rays  of  the 
triakisoctahedral  or  of  the  ikositetrahedral  skeleton. 

The  octahedral  silver  bromide  needles  thus  show  no  other 
directions  of  most  rapid  growth  than  have  already  been  found 
in  the  usual  octahedra  and  the  octahedral  plates. 


FIG.  51 

Diagram  showing  directions  of  most  intensive  growth 
of  an  octahedral  plate 

4.  In  other  crystal  forms  of  silver  bromide.  In  addition  to 
octahedra,  prisms  and  plates  of  pentagonal  dodecahedra  and 
of  rhombic  dodecahedra  have  been  found  in  silver  bromide. 
That  there  are  rhombic  dodecahedral  skeletons  with  rays 
which  coincide  with  the  sides  of  the  rhombus  could  not  be 
proved. 

The  pentagonal  dodecahedra,  which  appear  only  in  com- 
bination with  octahedra,  are  formed  in  large  numbers  when 
one  or  two  per  cent  gelatine  or  about  0 . 005  per  cent  of  agar- 
agar  is  added  during  the  process  of  precipitation  from  suddenly 
cooled,  highly  concentrated  solutions.  Skeletons  were  often 
observed  of  which  the  rays  coincided  with  the  sides  of  the 
pentagonal  pyramids  which  are  so  characteristic  in  these 
combinations.  The  rays  of  these  skeletons  formed  angles  of 
about  72°  with  each  other.  No  dimensions  can  be  given, 
as  it  was  impossible  to  measure  the  skeletons  on  account  of 
their  crude  formation. 

117 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

A  large  number  of  cubes  and  cubic  skeletons  were  observed, 
of  which  fifteen  different  forms  are  reproduced  in  Fig.  52. 
Here  combinations  with  the  octahedra  may  be  clearly  seen 
in  f  and  h.  Whether  these  were  plates  or  the  ordinary  crystals 
could  not  always  be  determined  with  certainty.  Presumably 
a  to  f  are  plates.  The  only  ones  that  could  be  positively 
identified  as  ordinary  crystals  are  g  to  p.  Cubes  could  readily 
be  identified  by  turning  them  over,  when  the  different  dimen- 
sions could  be  definitely  seen.  They  are  approximately 
mathematically  true  cubes. 

It  is  difficult  to  say  in  which  direction  the  most  rapid 
growth  of  fully  developed  cubes  takes  place.  Many  axial 
skeletons  were  observed  which  coincided  with  the  main  axes 
and  had  cubic  faces  at  the  ends.  This  may  be  the  case  with 
skeletons  which  coincide  with  the  diagonals  of  the  cube,  but 
so  far  such  skeletons  have  not  been  observed. 

The  most  rapid  growth  in  cubic  faces  is  clearly  shown  in 
the  skeletons  b  to  e  in  Fig.  52.  These  coincide  with  the 
diagonals  of  the  rectangle,  and  this  direction  is  also  shown  in 
the  laminated  structure  on  the  cube  surfaces  g  to  p;  so  that, 
presumably,  the  skeleton  was  formed  first,  and  then  developed 
into  the  complete  crystal.  The  surfaces  k  to  o  (Fig.  52)  show 
growth  also  in  directions  parallel  to  the  sides  of  the  cube. 
This  is  very  clearly  shown  in  Fig.  52m.  However,  these 
directions  are  the  same  as  those  already  mentioned,  which 
coincide  with  the  main  axes. 

It  is  obvious  that  the  direction  of  greatest  growth  of  cubic 
needles  coincides  with  one  of  the  main  axes.  A  cubic  needle 
without  an  octahedral  point,  although  not  impossible,  has  not 
yet  been  observed  in  silver  bromide.  But  the  fact  that 
these  cubic  needles  may  have  only  one  octahedral  point,  the 
other  being  (e.  g.)  cubical,  leaves  unsettled  the  question 
as  to  whether  the  growth  in  one  direction  is  to  be  attributed 
to  the  greater  octahedral  acumination. 

SUMMARY   OF   THE   OBSERVATIONS   CONCERNING   THE   DIRECTION 

OF    THE    MOST   RAPID    GROWTH   OF    SILVER 

BROMIDE   CRYSTALS 

The  principle  enunciated  by  Lehmann  that  the  directions 
of  greatest  development  are  generally  coincident  with  the  place 
of  greatest  acumination  of  the  crystals  may,  therefore,  not 
apply  to  all  silver  bromide  crystals. 

The  principle  applies  to  the  following  forms: 

Octahedra ; 

Octahedral  axial  skeletons; 

118 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 


FlG.  52 

Skeletons  and  cubes  of  silver  bromide.     A-B,  2500  diameters' 
magnification;  C-P,  800  diameters' 

119 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

Octahedral  plates  with  triakisoctahedral  skeletons; 
Triakisoctahedral  skeletons; 
Cubic  plate  skeletons; 
Cubic  plates. 

The  principle  as  definitely  does  not  apply  to  the  following 
forms : 

Octahedral  plates  with  ikositetrahedral  skeletons  (see  Fig.  48f) ; 
Ikositetrahedral  skeletons; 

Octahedral  plates  with  triakisoctahedral  and  ikositetrahedral  skeletons; 
Octahedral  plates  which  have  dyakisdodecahedral  skeletons. 

The  principle  is  doubtful  in  regard  to  cubes  and  cubic 
needles. 

A  law  concerning  the  direction  of  most  rapid  growth 
can  not  yet  be  formulated.  It  may  coincide  with  the  angles  of 
greatest  acumination,  or  it  may  coincide  with  the  corners  of 
more  obtuse  points,  or  even  with  the  normals  of  the  surfaces. 
That  the  capillary  constant  may  play  a  very  important  part 
here  has  already  been  intimated.  The  extreme  complexity 
of  the  problem  is  evident  from  the  fact  that,  in  many  cases,  a 
certain  direction  of  growth  may  suddenly  change  during  the 
process  of  crystal-formation  without  it  being  certain  that  this 
was  caused  by  a  modification  of  the  conditions  under  which 
the  crystallization  proceeded. 


120 


CHAPTER  X 

THE    BEHAVIOR    OF    SILVER    BROMIDE   AND   SILVER   IODO-BROMIDE 
CRYSTALS    IN    POLARIZED    LIGHT 

Since  the  polymorphism  of  silver  bromide  is  of  the  greatest 
importance  in  the  theory  of  the  photochemical  processes  in 
photographic  plates,  a  large  number  of  the  silver  bromide 
hexagons  were  microscopically  examined  in  polarized  light. 

If  these  hexagons  belong  to  the  hexagonal  crystal  system, 
they  can  not  be  other  than  a  combination  of  hexagonal  prisms 
with  the  pinacoid  as  basis.  Such  a  combination  crystal 
shows  simple  optical  refraction  only  in  the  direction  of  the 
principal  axis.  In  every  other  direction  it  is  said  to  show 
double  refraction  between  crossed  nicols.  In  fact  this  was 
observed  by  Elsden,  and  may  perhaps  be  the  reason  for  his 
mention  of  hexagonal  silver  bromide. 

Polarization  of  light  in  these  crystals  is,  however,  quite 
different  from  that  which  is  caused  by  anisotropism  of  the 
crystals  of  other  than  the  regular  system.  Higson  (1.  c.)  has 
obtained  only  negative  results  from  the  examination  of  crystals 
of  the  regular  system  in  polarized  light.  In  the  different 
crystal  plates,  which  are  of  various  thicknesses,  the  polarization 
is  always  approximately  the  same,  and  it  is  difficult  to  see  the 
characteristic  Newton  color  series.  This  phenomenon  shows 
clearly  that  here  one  is  dealing  with  polarization  due  to 
reflection,  and  not  with  double  refraction  produced  by  the 
crystal  structure.  This  reflection  is  very  strong  in  prepara- 
tions of  microscopic  crystals  mounted  in  Canada  balsam  on 
account  of  the  great  differences  in  the  refractive  indices.  For 
silver  bromide  at  X431,  n  =  2.360;  at  X656,  n  =  2.2336;1 
and  for  Canada  balsam,2  n  =  1 . 528  -  1 . 532. 3 

But  the  silver  iodo-bromide  crystals  of  photographic 
emulsions  behave  quite  differently.  Between  crossed  nicols 
they  show,  beside  the  unavoidable  reflections,  a  distinct  double 
refraction,  as  may  be  seen  in  Fig.  22.  This  figure  is  a  photo- 
micrograph of  exactly  the  same  area  of  the  same  emulsion  as 
that  in  Fig.  21,  and  is  taken  at  the  same  magnification. 

In  some  emulsions  even  a  colored  axial  figure  was  observed, 
which,  in  combination  with  the  cloud-like  distribution  of  the 

1  Landolt's  Physikalisch-chemische  Tabellen,  p.  983  (Fourth  Edition,  1912). 

2  Ibid.,  p.  981. 

3  It  would  be  simplest  to  examine  the  axial  image  in  convergent  polarized  light.    How- 
ever, on  account  of  the  difficulty  of  mounting  a  Bertrand  lens  in  a  microscope,  and  of 
obtaining  a  petrographic  microscope  during  the  war,  these  methods  were  dispensed  with. 

121 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

light  spots  in  the  crystal,  shows  that  here  one  is  dealing  not 
with  a  normal  crystallographic  anisotropism  of  silver  bromide, 
but  with  an  optical  anomaly  which  is  caused  by  tensions  in 
the  crystal  structure.  This  has  no  connection  with  the 
polymorphism  of  silver  bromide.  It  shows  only  that  the 
crystalline  structure  of  silver  bromide  in  photographic  emul- 
sions is  more  complicated  than  that  of  silver  bromide  alone, 
not  in  so  far  as  that  one  has  to  do  with  another  lattice  struc- 
ture, but  rather  that  the  crystal  lattice  contains  more  or  less 
regularly  distributed  foreign  bodies  which  greatly  affect  the 
optical  properties  of  silver  bromide  and,  as  Reinders1  has 
demonstrated,  probably  exert  a  very  great  influence  on  the 
light-sensitivity  of  the  bromide. 

The  above-mentioned  examination  shows  that  at  present 
we  know  only  regular  silver  bromide  in  the  stable  phase  at 
regular  temperatures.  This  greatly  simplified  the  present 
work,  as  it  made  it  possible  to  distinguish  the  different  crystals 
direct  without  troublesome  goniometric  measurements.  Vari- 
ous necessary  measurements, — e.  g.,  of  the  silver  bromide  skel- 
eton— are  not  possible  on  account  of  the  small  size  of  the 
crystals  (10-30^)  on  the  one  hand,  and  the  impossibility  of  ob- 
serving the  same  crystal  in  different  positions  on  the  other. 
One  method,  to  obtain  the  crystals  suspended  in  different  posi- 
tions in  gelatine,  does  not  give  satisfactory  results  on  account 
of  the  large  aberration  due  to  the  depth  of  the  necessary  opti- 
cal system,  and  the  great  diversity  of  the  skeletons. 

THE    ANOMALOUS    OPTICAL    ACTIVITY    OF    SILVER    IODO-BROMIDE 
CRYSTALS   IN   PHOTOGRAPHIC   EMULSIONS 

Only  two  methods  are  known  by  which  it  is  possible  to 
change  a  mono-refractive  medium  into  a  doubly  refractive 
one:  a)  by  placing  the  medium  in  an  electric  or  magnetic 
field  of  high  intensity;  and  b)  by  subjecting  the  medium  to 
an  internal  or  external  mechanical  strain. 

The  first  method  is  practically  impossible  in  the  case  of 
the  silver  halide  crystals  of  photographic  emulsions.  Even 
if  there  is  a  contact  potential  between  the  silver  halide  and 
the  gelatine,  it  can  not  be  more  than  a  few  volts,  which  is 
much  too  low  to  produce  double  refraction  between  crossed 
nicols.  For  the  present,  then,  we  must  attribute  the  double 
refraction  of  the  silver  halide  crystals  to  mechanical  strain. 

Brauns2  has  shown  that  alum  crystals  are  mono-refractive 
only  when  they  are  chemically  pure,  and  that  isomorphic 
mixtures  are  made  doubly  refractive  by  even  a  slight  content 

1  Reinders,  W.,  1.  c. 

2  Brauns,  R.,  cited  by  Groth  in  Physikalische  Krystallographie,  p.  513. 

122 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 


of  another  alum.  Because  of  the  different  intervals  between 
the  alternately  superimposed  atoms  in  the  crystal  structure, 
an  internal  tension  is  produced  which  causes  double  refraction. 

Later  the  same  phenomenon  was  observed  in  other  salts. 

In  view  of  the  fact  that  the  silver  halide  crystals  of  photo- 
graphic emulsions  may,  within  certain  limits,  be  considered 
as  isomorphous  mixtures  of  regular  silver  iodide  and  silver 
bromide,  the  anomalous  double  refraction  may  be  partly 
accounted  for. 


FIG.  53A 

Silver  iodo-bromide  emulsion  between  crossed  nicols,  showing  the 
effect  of  mechanical  strain  in  the  gelatine  around  the  grains 

In  order  to  prove  this  experimentally,  silver  iodide  was 
added  to  an  ammoniacal  solution  of  silver  bromide.  The 
solution  was  shaken  frequently  until,  after  24  hours,  it  was 
supposed  that  equilibrium  between  the  solid  and  the  liquid 
phase  was  established.  5  cc.  of  this  solution  were  mixed  with 
40  cc.  of  water  heated  to  95°  C.,  and  cooled  in  a  closed  bottle 
set  in  ice-water  (temperature  4°  C.).  The  crystals  formed 

123 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

were  washed,  put  on  a  slide  in  Canada  balsam,  and  covered 
with  a  cover-glass.  The  anomalous  double  refraction  between 
crossed  nicols  is  very  distinct  in  Fig.  53a,  which  is  a  photo- 
micrograph of  this  preparation  at  500  diameters'  magnification. 
However,  this  does  not  quite  solve  the  problem,  because  pure 
silver  bromide  emulsions,  specially  prepared  in  this  laboratory, 
also  show  polarization  phenomena  between  crossed  nicols. 

It  is  possible  that  in  the  drying  of  the  plates  the  surround- 
ing gelatine  exerts  a  pressure  on  the  silver  halide  grains.    This 


FIG.         53B 

Silver  iodo-bromide  emulsion  between  crossed  nicols,  showing  the  effect 
of  mechanical  strain  in  the  gelatine  around  the  grains.  It  is  possible  that 
in  this  case  these  radial  effects  are  due  to  scattering  of  light  reflected  by 
the  flat  surfaces  of  the  crystals,  which  is  not  the  case  in  emulsion  grains. 

external  strain  should  be  present  in  the  gelatine  around  the 
crystals,  and  should  be  visible  between  crossed  nicols.  Indeed, 
faint  indications  of  this  were  noticed  in  very  coarse-grained 
emulsions,  such  as  that  shown  in  Fig.  22  at  1,350  diameters' 
magnification.  Therefore,  to  make  fhe  phenomenon  more 
visible,  some  emulsions  were  dried  more  rapidly  and  at  a 

124 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

higher  temperature.  The  result  is  shown  in  Fig.  53b.  There 
are,  therefore,  external  strain  lines  which  radiate  from  the 
silver  halide  grains  into  the  colloidal  matrix.  But  the  silver 
halide  grains  and  even  the  silver  bromide  grains  of  photo- 
graphic emulsions  still  showed  anomalous  polarization  phe- 
nomena between  crossed  nicols  when  they  were  freed  from 
the  surrounding  gelatine  by  boiling  with  dilute  sulphuric  acid. 
So  this  experiment  presented  no  definite  solution  of  the 
problem. 

Therefore,  the  only  remaining  explanation  is  in  the  struc- 
ture of  the  silver  halide  grains. 

That  the  grains  of  photographic  plates  do  not  consist  of 
pure  silver  halide,  but  of  a  system  of  gelatine  and  silver  halide 
(plus  water  plus  salts?),  is  a  thesis  which  has  already  been 
discussed.  (Chapter  III.)  Quincke1  advanced  a  theory 
concerning  the  colloid-chemical  structure  of  silver  halide 
grains  in  photographic  emulsions.  Bellach2  found  that 
careful  drying  sometimes  reduces  the  average  size  of  the 
grains.  When  testing  one  of  Eder's  emulsions  he  found  a 
contraction  of  0.65  x  10~5  sq.  mm.  per  grain,  which  indicates 
a  complex  structure.  Hodgson,3  however,  could  not  observe, 
even  at  the  highest  magnification,  any  swelling  of  the  grains 
wrhen  soaked  in  water.4  This,  however,  does  not  contradict 
Bellach 's  observation,  in  view  of  the  fact  that  for  changes 
in  volume  of  the  silver  halide  grains,  riot  only  is  the  presence 
of  gelatine  essential,  but  the  manner  in  which  it  is  distributed 
in  the  grains  is  also  of  importance.  If  there  is  a  regular 
distribution  of  hermetically  sealed  gelatine  particles  in  the 
grains,  their  swelling  is  not  to  be  expected  under  any  circum- 
stances. If,  on  the  other  hand,  there  is  a  continuous  network 
of  gelatine  in  the  grains,  it  is  quite  possible  to  effect  a  change 
of  volume  if  the  internal  gelatine  is  in  contact  with  the  external 
gelatine,  and  if  the  elasticity  of  the  layers  of  the  silver  bromide 
crystals  resisting  displacement  is  not  too  great.  The  greater 
the  resistance  of  the  silver  bromide  to  changes  in  volume, 
the  greater  the  internal  tension  in  the  grains. 

With  the  recognition  of  the  crystalline  structure  of  silver 
halide  grains  in  photographic  emulsions,  the  conception  that 

1  Quincke,  G.,  Niederschlagmembranen  und  Zellen  in  Gallerten  oder  Losungen  von 
Leim,    Eiweiss,   und   Starke.     Ann.    Physik.     IV.     11:  449.      1903.     Die   Bedeutung   der 
Oberflachenspannung  fur  die  Photographic  mit  Bromsilbergelatine.     Ibid.     IV.     11:1100. 
1903. 

2  Bellach,  V.,  1.  c. 

3  Hodgson,  M.  B.,  1.  c. 

4  We  repeated  Hodgson's  experiment  at  a  magnification  of  2,500  diameters,  using  the 
emulsion  pictured  in  Fig.  21.     This  emulsion  was  poured  out  in  a  very  thin  layer  (of  one 
grain  in  thickness)  and  dried.     The  emulsion  side  was  turned  toward  the  condenser  and  a 
photomicrograph  of  the  grains  taken.     If  the  condenser  is  removed  very  carefully,  one  can 
moisten  the  emulsion  and  again  photograph  it.     This  procedure  showed  no  difference  in 
the  dimensions  of  the  grains  in  the  dry  and  the  wet  emulsion. 

125 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

these  crystals  are  a  binary  system  disappeared.  Luppo- 
Cramer1  expressed  himself  differently  at  different  times 
regarding  the  structure  of  the  silver  halide  grains.  First, 
he  assumed  that  the  crystals  are  free  from  gelatine.  In  his 
later  work,  however,  he  vigorously  and  often  defends  the 
theory  of  the  heterogeneity  of  silver  halide  grains  in  photo- 
graphic emulsions.  So  he  probably  differentiates  between 
two  different  kinds  of  silver  halide  grains  in  emulsions:  crystal- 
line grains  and  colloidal  grains.  W.  D.  Bancroft2  is  also  of 
the  opinion  that  the  whole  question  of  the  high  sensitivity  of 
photographic  plates  depends  on  the  system  silver  halide- 
gelatine. 

The  possibility  of  gelatine  in  silver  halide  crystals  was 
first  demonstrated  experimentally  by  Reinders.3  He  showed 
that  silver  chloride,  crystallizing  in  the  presence  of  various 
colloids,  such  as  colloidal  silver,  gelatine,  albumen,  casein, 
etc.,  has  the  capacity  of  taking  up  and  homogeneously  distrib- 
uting these  colloids  in  the  crystals.  With  gelatine  this  effect 
was  noticeable  even  in  concentrations  of  1  mg.  gelatine  in  10 
liters  of  water.4 

If  the  anomalous  double  refraction  of  silver  halide  crystals 
is  caused  by  this  enclosed  gelatine,  then  silver  bromide  crystals 
from  ammoniacal  silver  bromide  solutions  containing  gelatine 
should  show  double  refraction,  since  colloid-free  silver  bromide 
solutions  apparently  can  yield  only  simply  refractive  crystals. 
But  no  definite  double  refraction  could  be  detected 
experimentally. 

However,  this  experiment  is  not  conclusive,  because  the 
conditions  of  crystallization  in  photographic  emulsions  are 
entirely  different  from  those  in  ammoniacal  solution.  In 
photographic  emulsions  there  is,  first,  a  suspension  of  colloidal 
silver  halide  which  consists  of  more  or  less  fine  flakes  of  silver 
halide  of  varying  size  and  structure.  The  diameters  vary 
from  submicroscopic  to  dr  I/*.  In  the  presence  of  a  silver 
halide  solvent  (such  as  potassium  bromide  or  ammonia),  the 
colloidal  silver  halide  passes  over  into  the  more  stable  crystal- 
line silver  halide.  Gelatine  is  continually  absorbed  during 
the  formation  of  the  crystal,  throughout  which  it  is  homo- 
geneously distributed  (Reinders). 

But  the  silver  halide  flakes  also  contain  some  microhetero- 
geneously  distributed  particles  of  gelatine  which  remain  in 

1  Luppo-Cramer,  Photographische  Probleme,  1.  c. 

2  Bancroft,  W.  D.,  The  photographic  plate,  1.  c. 

3  Reinders,  W.,  Studien  iiber  die  Photohaloide,  1.  c. 

4  This  visible  decomposition  is  distinct  from  the  latent  effect  of  light.     Therefore,  the 
laws  concerning  visible  photochemical  decomposition  can  not  be  applied  to  the  latent  effect 
of  light. 

126 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

the  crystal  and  form  a  structure  different  from  that  described 
by  Reinders.  The  grains  of  sensitive  plates  have  a  diameter 
of  2  to  3/^  and  more.  Hence  there  must  be  another  factor 
concerned  than  the  simple  conversion  of  flakes  into  crystals. 
The  increase  in  size  of  the  grains  may  be  due  either  to  the 
combining  of  several  single  flakes  during  the  crystallization 
process,  or  to  the  crystallization  of  smaller  flakes  on  larger. 
However  this  development-process  is  regarded,  the  gelatine 
structure  of  the  flakes  is  not  necessarily  lost  in  crystallization, 
but  may  pass  over  into  the  crystals.  There  may  be  a  change 
in  structure,  depending  upon  the  treatment  of  the  emulsion. 
In  fact,  the  gelatine  structure  depends  upon  the  conditions 
of  the  precipitation  and  the  subsequent  treatment  of  the 
emulsion.  (Hodgson  vs.  Bellach.)  This  microheterogeneous 
structure,  which  does  not  occur  when  silver  bromide  is  precip- 
itated from  ammoniacal  gelatine-containing  solutions,  may  be 
the  cause  of  the  tensions  in  the  emulsion  crystals,  to  which 
the  anomalous  double  refraction  is  to  be  ascribed. 

If,  however,  a  mono-refractive  medium  contains  micro- 
scopic or  submicroscopic  suspensions  of  dielectrics,  the  double 
refractivity  may  not  be  the  only  cause  of  the  abnormal  phe- 
nomenon seen  between  crossed  nicols.  Diffuse  reflections 
can,  under  these  conditions,  illuminate  the  dark  field.  But 
it  is  hardly  conceivable  that  this  may  produce  polarization 
phenomena  which  resemble  the  axial  images  sometimes  found 
in  emulsions.  However,  these  images  are  possible  if  strains 
are  present. 

Thus  we  must  conclude  that  the  anomalous  optical  activity 
of  silver-iodo-bromide  crystals  in  photographic  plates  may 
be  the  result  of  any  of  several  factors : 

a.  The  tensions  resultant  from  the  isomorphic  mixture  of  silver  iodide 
and  silver  bromide  (cf .  Chapter  V) ; 

b.  The  mechanical  strain  exerted  to  a  small  degree  by  the  gelatine  sur- 
rounding the  silver  iodo-bromide  crystals; 

c.  The  probable  mechanical  strain  of  microheterogeneously  distributed 
gelatine  particles  in  the  silver  halide  crystals; 

d.  The  probable  diffuse  reflections  in  microheterogeneously  suspended 
gelatine  in  the  silver  halide  crystals. 

Hence  we  must  regard  sensitive  photographic  emulsions 
not  only  as  suspensions  of  silver  halide  crystals  in  gelatine, 
but  also  as  a  probably  very  much  more  complex  suspension 
of  gelatine  in  crystalline  silver  halide.  A  fuller  discussion 
of  this  aspect  of  the  question  has  been  given  in  Chapter  IV. 

The  probable  complexity  of  this  suspension  has  been 
indicated  by  a  distinction  between  a  homogeneous  and  a 

127 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

microheterogeneous  suspension,  which  refers  only  to  a  different 
degree  of  distribution.  Therefore,  these  terms  should  be 
interpreted  only  in  a  relative  sense.1  As  a  matter  of  fact, 
both  types  of  distribution  lie  beyond  the  limits  of  microscopic 
resolving  power.  Therefore,  both  could  be  interpreted 
equally  well  as  homogeneously  distributed  gelatine.  Gibbs 
has  emphasized  the  fact  that  the  conceptions  "homogeneous" 
and  "heterogeneous"  are  relative.  That  is  to  say,  one  can  not 
make  a  sharp  distinction  between  the  two  because  there  is  a 
continuous  transition  between  homogeneous  and  hetero- 
geneous, according  to  the  point  of  view.  Take,  for  example, 
the  photographic  emulsion.  Microscopically  considered,  it 
is  heterogeneous;  but  macroscopically  it  is  homogeneous. 
The  purest  crystal  medium  is  homogeneous  in  ordinary  light 
but  heterogeneous  in  the  X-ray.  Hence  the  grades  of  distrib- 
ution of  gelatine  in  silver  halide  crystals,  as  mentioned  above, 
can  be  interpreted  as  both  homogeneous  and  heterogeneous, 
or  one  as  homogeneously,  and  the  other  as  heterogeneously 
distributed,  according  to  one's  point  of  view. 

THE    GELATINE   ENCLOSING   THE    SILVER   IODO-BROMIDE 
CRYSTALS    OF   PHOTOGRAPHIC   EMULSIONS 

The  radial  distribution  of  the  tension  exerted  by  the 
gelatine  around  the  crystals  in  photographic  emulsions  indi- 
cates a  complex  effect.  This  appears  not  only  in  gelatine, 
but  also,  as  Fig.  53B  shows,  in  Canada  balsam.  This  balsam 
is  a  very  viscous  liquid,  which  makes  the  phenomenon  appear 
even  more  complex.  The  cause  is  unknown.  Possibly  the 
following  observations  may  suggest  a  solution. 

In  some  preliminary  experiments  on  the  adsorption  of 
colloids  by  silver  bromide  crystals,  colloidal  dyes  were  used. 
If  the  entire  process  of  crystallization  takes  place  in  the 
presence  of  such  dyes,  then  the  Reinders'  distribution  of 
colloid  in  crystal  can  be  directly  (microscopically)  proven.2 
An  analogous  phenomenon  was  noticed,  especially  with  the 
sodium  salt  of  l-naphthol-4-sulphonic  acid-azo-/3-napthol. 
An  excess  of  this  dye  coagulates  around  the  silver  bromide 
crystal  in  the  form  of  rays  which  may  vary  greatly  under 
different  conditions. 

For  gelatine,  therefore,  the  phenomenon  may  be  explained 
as  follows:  By  a  ray-like  coagulation  of  the  gelatine  around 

1  Where  differences  in  space  distribution  of  the  same  kind  of  material  are  concerned, 
the  terms   "iso-psegmatic"    (equal-grained)    and    "allo-psegmatic"   or      poly-psegmatic 
(vari-grained)  are  more  suitable. 

2  Cf.  also  Marc's  experiments,  cited  on  page  52. 

128 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 

the  grains,  spots  of  greater  or  less  density  occur  in  the  gelatine. 
The  drying  of  the  gelatine  has  a  different  effect  in  different 
places  because  of  this  structure,  as  the  rays  will  tend  to  dry 
more  quickly  than  the  rest  of  the  gelatine.  All  strain  will  be 
neutralized  by  internal  displacements  if  the  drying  is  slow. 
But  if  the  drying  is  rapid,  there  is  not  sufficient  time  to  com- 
pensate the  resultant  strain,  and  a  double  refraction  of  gelatine 
may  be  seen  through  crossed  nicols. 

This  presupposes  that  gelatine  possesses  a  solidification 
structure  which  persists  in  dry  gelatine,  and  which  originates 
in  a  manner  analogous  to  that  of  the  solidification  of  liquids 
to  crystalline  aggregates  in  which  radially  arranged  structures 
(spherolites)  may  be  observed.  Still  this  leaves  unexplained 
the  same  phenomenon  in  the  viscous  Canada  balsam.  Here, 
however,  one  is  entering  the  province  of  pure  colloid-chemical 
investigation,  which  is  outside  the  scope  of  the  present 
discussion. 

GENERAL  SUMMARY  OF  THE  CRYSTALLOGRAPHIC  STUDY  OF 
SILVER  BROMIDE  CRYSTALS 

1.  It  has  been  shown  that  silver  iodide  is  precipitated  from 
ammoniacal  solutions  in  the  metastable  regular  form,  which  is 
isomorphous  with  silver  bromide,  and  that  the  crystallographic 
classification  of  silver  bromide  may  apply  also  to  the   silver 
iodo-bromide  crystals  of  photographic  emulsions. 

2.  The  occurrence  of   silver  bromide   pseudomorphs   was 
investigated,  and  methods  for  recognizing  silver  bromide-am- 
monia complexes  described. 

3.  All  the  crystalline    forms   of   silver  bromide   obtained 
were  identified. 

4.  The  faces  of  needles  and  plates  of  silver  bromide  were 
studied,  and  shown  to  be  growth-modifications  of  octahedra 
and  cubes. 

5.  Etch-figures  were  obtained  on  the  octahedral  faces  of 
silver  bromide. 

6.  Silver  bromide  was  assigned  to  the  dyakisdodecahedral 
class  of  the  regular  system. 

7.  Because  of  their  isomorphism,  silver  chloride  and  regular 
silver  iodide  were  placed  in  the  same  class  as  silver  bromide. 

8.  The    possibilities    of    modifications    of   silver    bromide 
were  discussed. 

9.  The  occurrence  of  needles,  plates,  and  ordinary  crystals 
of     silver     iodo-bromide     in     photographic     emulsions     was 
demonstrated. 

129 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

10.  It    was    shown    that    the    Gibbs-Curie-Wulff    law    is 
applicable  to  photographic  emulsions,  and  that  thereby  all 
octahedral    forms    appearing    in    these    emulsions    may    be 
explained. 

11.  The  directions  of  most  rapid  growth  were  determined 
for  the  various  crystalline  forms  of  silver  bromide. 

12.  It  was  shown  that  Lehmann's  theory  concerning  the 
direction   of   most   rapid   development   of   crystals   does   not 
always  apply  to  silver  bromide  crystals. 

13.  The  examination  of  the  characteristic  optical  properties 
of  silver  bromide  crystals  in  polarized  light  showed  that  the 
so-called  hexagonal  silver  bromide  is  really  regular. 

14.  The  possible  causes  of  the  anomalous  optical  activity 
of  silver  halide  crystals  of  photographic  emulsions  in  polarized 
light  were  studied,  and  it  was  suggested  that  these  crystals 
are  the  center  of  various  mechanical  strains. 

15.  It  was  suggested  that  there  are  two  different  gelatine 
structures  in  the  crystals  of  photographic  emulsions. 

16.  The  structure  of  the  gelatine  surrounding  the  grains 
in  photographic  emulsions  was  more  closely  studied  and  a 
radial  coagulation  of  gelatine  around  the  grains  observed. 

In  conclusion  we  wish  to  express  our  great  appreciation 
of  the  co-operation  of  Messrs.  W.  H.  Davis,  F.  A.  Elliott, 
G.  H.  Norris  and  L.  Schneider,  who  assisted  in  the  experi- 
mental part  of  this  investigation,  and  of  Professor  A.  C.  Gill, 
of  Cornell  University,  who  read  the  manuscript  and  made  help- 
ful criticisms  and  valuable  suggestions. 


130 


SILVER  BROMIDE  GRAIN  OF  PHOTOGRAPHIC  EMULSIONS 


CONCORDANCE   OF   POSSIBLE    SYMMETRY   CLASSES 
OF    SILVER   BROMIDE 

Inasmuch  as  the  forms  included  in  the  regular  system,  to 
which  silver  bromide  belongs,  are  variously  named  by  different 
authorities,  a  correlation  of  the  terminology  is  given  below: 


Authorities 
Hilton 
Symbol       Schonfliess 

T       Tetartrohedry 


Th      Paramorphic 
hemihedry 

Central 


Td     Hemimorphic  XXXI 

hemihedry  Ditesseral 

Polar 


O       Enantiomorphous       XXIX 
hemihedry  Tesseral 

holoaxial 


H.  A. 
Miers 

E.  S. 
Dana 

Number  of 
faces  in 
Other                    general 
Names                     form 

XX-VIII 

Tesseral 
Polar 

5  Tetartro- 
hedral 

Pentagonal 
dodecahedral 

12 

XXX 

Tesseral 

2  Pyrito- 
hedral 

Parallel  faced 
hemihedry 

24 

Pentagonal 
hemihedry 
Dyakisdodecahedral 


Oh     Holohedry 


XXXII 

Ditesseral 
Central 


3  Tetrahedral 


4  Plagihedral 


1  Normal 


Hexakis- 
tetrahedral 
Inclined  faced 
hemihedry 

Gyroidal 
hemihedry 
Pentagon- 
ikositetrahedral 

Hexakis- 
octahedral 


24 


24 


48 


131 


MONOGRAPHS  ON  THE  THEORY  OF  PHOTOGRAPHY 

The  Silver  Bromide  Grain 

Abbreviations  adopted  in  citations  of  serial  publications. 

Astrophys.  J The  Astrophysical  Journal 

Arch.  wiss.  Phot.    .      .      .      .      Archiv  fiir  wissenschaftliches  Photographic 

Ann.  Physik Annalen  der  Physik 

Ann.  chim.  phys Annales  de  chimie  et  de  physique 

Ber.  chem.  Gesell.        .      .      .      Berichte  der  deutschen  chemischen 

Gesellschaft 
Bull.  soc.  franc.,  mineral.        .      Bulletin  de  la  societe  francaise  de 

mineralogie 

Brit.  J.  Phot The  British  Journal  of  Photography 

Brit.  J.  Phot.  Almanac     .      .      The     British     Journal    of     Photography 

Almanac 

Fortschr.  Mineral.       .      .      .      Fortschritte    der    Mineralogie,     Krystal- 

lographie,  und  Petrographie 

J.  Amer.  Chem.  Soc.  .      .      .      Journal  of  the  American  Chemical  Society 
J.  Amer.  Leather  Chem.  Assoc.  Journal  of  the  American  Leather  Chemists' 

Association 
J.  Chem.  Soc.  (Trans.)      .      .      Journal  of  the  Chemical  Society 

(Transactions) 
J.  chim.  phys.         ....      Journal  de  chimie  physique 

J.  Frankl.  Inst Journal  of  the  Franklin  Institute 

J.  Phys.  Chem The  Journal  of  Physical  Chemistry 

J.  Soc.  Chem.  Ind.       .      .      .      Journal  of  the  Society  of  Chemical 

Industry 
J.  Wash.  Acad.  Sci.     .      .      .      Journal  of  the  Washington  Academy  of 

Science 
Jahrb.  Phot Jahrbuch    fiir    Photographic   und    Repro- 

ductionstechnik  (Eder's) 
Jahrb.  Rad.  u.  Elektr.      .      .      Jahrbuch  der  Radioaktivitat  und 

Elektronik 

Koll.-Zeits Kolloid-Zeitschrift 

Neues  Jahrb.  Mineral.  Geol.        Neues  Jahrbuch  fiir  Mineralogie,  Geologic 

und  Palaontologie 
Phil.  Mag The     London,     Edinburgh     and     Dublin 

Philosophical  Magazine  and  Journal  of 

Science 
Phil.  Trans Philosophical  Transactions  of  the  Royal 

Society  of  London 

Phot.  J The  Photographic  Journal 

Phot.  News Photographic  News 

Physik.  Zeits.          ....      Physikalische  Zeitschrift 

Sitzungsber.  Akad.  Wiss.  Wien      Sitzung§berichte  der  kaiserlichen 

Akademie  der  Wissenschaften,  Wien 

Zeits.  anorg.  Chem.     .      .      .      Zeitschrift  fiir  anorganische  und 

allgemeine  Chemie 
Zeits.  Kryst.  u.  Mineral.         .      Zeitschrift  fiir  Krystallographie   und 

Mineralogie 
Zeits.  physik.  Chem.          .      .      Zeitschrift     fur     physikalische     Chemie, 

Stochiometrie  und  Verwandtschaftslehre 
Zeits.  wiss.  Phot.          .      .      .      Zeitschrift  fiir  wissenschaftliches 

Photographic 

132 


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SCHEFFER,  W.,  Microscopical  researches  on  the  size  and  distribution  of 

the  plate  grains.     Brit.  J.  Phot.  54:  116.   1907. 
SHEPPARD,  S.  E.,  Photochemistry.     (Longmans,  1914.) 

— ,  — .  — .,  and  MEES,  C.  E.  K.,  Investigations  on  the  theory  of 

the  photographic   process.     (Longmans,    1907.) 

— ,  — .  — .,  and  MEYER,  G.,  Chemical  induction  and  photographic 

development.     Phot.   J.   60:   12.  1920;  J.  Amer.  Chem.   Soc.  42:  689 

1920. 
SMITS,  A.,  Eine  neue  Theorie  der  Erscheinung  Allotropie.     Zeits.  physik 

Chem.     76:421.  1911. 
STARK,  J.,  Zur  Energetik  und  Chemie  der  Bandenspektra.     Physik.  Zeits. 

9:  85.   1908. 
STAS,  J.  S.,  Recherches  de  statique  chimique  au  sujet  du  chlorure  et  du 

bromure  d'argent.     Ann.  chim.  phys.     V.  3:  145.  1874. 
STEUBING,  W.,  Fluoreszenze  und  lichtelektrische  Empfindlichkeit  anor- 

ganischer  Substanzen.     Physik.  Zeits.     9:493.1908. 
STOLTZENBERG,  H.,  and  HUTH,  M.  E.,  Ueber  kristallinisch-fliissige  Phasen 

bei  den  Monohalogeniden  des  Thalliums  und  Silbers.     Zeits.  physik. 

Chem.     71:  641.   1910. 
TAMMANN,    G.,    Das   Zustandsdiagramm   des   Jodsilbers.     Zeits.    physik. 

Chem.     75:  733.   1911. 

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THIEL,  A.,  Umkehrbare  Elektroden  zweiter  Art  mit  gemischten   Depo- 

larisatoren.     Zeits.    anorg.    Chem.     24:  1.   1900. 
TUBANDT,  C.,  and  LORENZ,  F.,  Das  elektrische  Leitvermogen  als  Methode 

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physik.  Chem.     87:  543.   1914. 

VOGEL,  H.  W.,  Handbuch  der  Photographie.     (Oppenheim,  Berlin,  1890.) 
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136 


Index  of  Authors 


ABEGG,  R.,  106 

BAKHUIS-ROOZEBOOM,  79 

BANCROFT,  W.  D.,  45,  48,  56,  61, 

106,  126 

BANKS,  E.,  61,  75 
BARMHAUER,  H.,  70 
BAUR,  E.,  80 
BECKE,  F.,  72 
BECKENKAMP,  J.,  50 
BELLACH,  V.,  61,  76,  80,  125,  127 
BEMMELEN,  J.  M.  VAN,  29 
BODLANDER,  G.,  and  FITTIG,  R.,  87 
BORNSTEIN,  R.,  LANDOLT,  H.  and 

MEYERHOFFER,  W.,  121 
BOWERMAN,  40 
BOWMAN,  J.  H.,  39 
BRAUNS,  R.,  41,  122 
BROTHER,  G.  H.,  39 
BURGESS,  C.  H.,  and 

CHAPMAN,  D.  L.,  48 
CHADWICK,  S.,  CHAPMAN,  D.  L., 

and  -      — ,  48 
CHAPMAN,  D.  L.,  BURGESS,  C.  H., 

and  -       — ,  48 

— ,   CHADWICK,  S.,  and 

RAMSBOTTOM,  J.  E.,  48 

— ,  and  MACMAHON,  P.  S.,  48 
CURIE,  P.,  57 
DUHEM,  P.,  43 
DYER,  61 
EDER,  J.   M.,    11,   13,    17,  25,   28, 

37,  45 

ELSDEN,  J.  V.,  80,  89,  121 
ENGLISCH,  E.,  12 
EPHRAIM,  F.,  25,  88 
FITTIG,  R.,  BODLANDER,  G., 

and  -      — ,  87 
FREUNDLICH,  H.,  66 
FRIEDEL,  G.,  58 
GAEDICKE,  J.,  11 
GIBBS,  J.  W.,  41,  57,  128 
GMELIN-KRAUT,  79 
GROSS,  R.,  59 

GROTH,  P.,  81,  97,  98,  107,  108,  122 
GROTTHUS,  T.  F.  VON,  48 
HIGSON,  G.  I.,  81,  100,  121 
HILTON,  H.,  58,  66 
HODGSON,  M.  B.,  84,  90,  125,  127 
HULETT,  G.  A.,  59 
HUTH,  M.  E.,  STOLTZENBERG,  H., 

and  —      — ,  46 
JOHNSEN,  A.,  68,  71 
JOHNSTON,  J.,  11 
KNOCHE,  P.,  LEHMANN,  E., 

and  -      — ,  106 


KOCH,  P.  P.,  and  DU  PREL,  G., 

93,  104 

KROHN,  F.  W.  T.,  75,  81,  93 
KUESSNER,  H.,  66,  67 
KUSTER,  F.  W.,  43 
LANDOLT,  H.,  BORNSTEIN,  R.,  and 

MEYERHOFFER,  W.,  121 
LANGMUIR,  I.,  30,  49 
LEHMANN,  E.,  and  KNOCHE,  P.,  106 
LEHMANN,  O.,  41,  107,  108,  118 
LEWIS,  W.  C.  M.,  48 
LORENZ,  F.,  TUBANDT,   C.,  and 

— ,  46,  47 
LORENZ,  R.,  84 
LUPPO-CRAMER,  12,  13,  17,  20,  22, 

23,  25,  27,  35,  48,  62,  76,  78,  91, 

106,  126 

LUTHER,  R.,  106 
MACMAHON,  P.  S.,  CHAPMAN,  D.  L., 

and  -          — ,  48 
MARBACH,  98 
MARC,  R.,  49,  52,  53 

— ,  and  RITZEL,  A.,  59 
MEES,  C.  E.  K.,  27,  33; 

SHEPPARD,  S.  E.,  and 

— -i — ,  11,  35,  53 
MEYER,  G.,  SHEPPARD,  S.  E.,  and 

— ,  66 
MEYERHOFFER,  W.,  LANDOLT,  H., 

BORNSTEIN,  R.,  and  —       — ,121 
MIERS,  H.  A.,  108 

M6NKEMEYER,  K.,  46 

MUGGE,  O.,  69,  72 
NERNST,  W.,  31 

NlEUWENBURG,   J.   VAN,    REINDERS, 

W.,  and  -          — ,  35 
NIGGLI,  P.,  68 
OSTWALD,  W.,  59,  66,  68 
PAWLOW,  P.,  68 
PREL,  G.  DU,  KOCH,  P.  P.,  and 

— ,  93,  104 
QUINCKE,  G.,  27,  125 
RAMSBOTTOM,     J.     E.,     CHAPMAN, 

D.  L.,  CHADWICK,  S.,  and 

AO 

,   ^O 

REINDERS,  W.,  49,  53 

—  and  VAN  NIEUWENBURG, 
J.,  35 

RENWICK,  F.  F.,  79,  81 
RETGERS,  J.  W.,  49,  52 
RIECKE,  E.,  41 
RITZEL,  A.,  66 
ROSCOE,  H.  E.,  and 

SCHORLEMMER,  C.,  80 


137 


SCHEFFER,  W.,  84  TAMMANN,  G.,  28,  47 
SCHORLEMMER,  C.,'  RoscoE,  H.  E.      TniEL,  A.,  44,  45  47   79  97 

S^^^TA  51  T™Tz  CF    46'  47' 

-3717  and  MEES'  C  E*  K"  "'      VALETO'J   J    59 

'- and  MEYER,  G.,  66  WALLACE,  R.,  91 

SMITS,  A.,  72  WEIMARN,  P.  VON,  28,  29,  30,  32 

STARK,  J.,  48  33,  68 

STAS,  J.  S.,  28  WILDERMAN,  M.,  31 

STEUBING,  W.,  48  WOOD,  R.  W.,  49 

STOLTZENBERG,  H.,  and  WYCKOFF,  R/W.  G    50 

HUTH,  M.  E.,  46  WULFF,  G.,  57,  58,  66   108 


138 


Index  of  Subjects 


Allotropy,  Smits'  theory  of,  72 

Ammonia,  complexes  of  silver  halides  and ,  16,  25,  86,  87;  in 

reversal,  21 
development,  effect  of  moisture  in,  18,  19 

-   of  latent  image  by,  11,  12,  18 
—  -   of  visible  image  by,  13 

— ,  rate  of,  22 
— ,  theory  of,  22 

exposure  necessary  for  development  with,  18 
fuming  of  exposed  plates,  18 

—  unexposed  layers  with,  13 

—  with,  in  absence  of  light,  22 

—  with,  after  destruction  of  the  image,  23 
use  of ,  in  ripening  emulsions,  11 

— ,  in  preparation  of  silver  bromide  crystals,  82,  85 
Anisotropism,  of  silver  bromide  crystals,  121,  122 
Apparatus  used  in  microscopic  study  of  silver  bromide,  82 

Bancroft's  theory  of  ripening,  56 
Beckenkamp's  theory  of  crystallization,  50 
Bellach's  study  of  ripening,  61 
Birefringence  in  crystals  of  emulsions,  47 

in  uniaxial  crystals,  69 
Bowerman's  principle  of  crystal  growth,  40 

Capillarity,  and  crystal  growth,  42,  57,  101,  103 
constants,  calculation  of,  58 
influence  of,  on  crystal  form,  42,  57,  100,  103 
Capstan0  effect,  53,  54,  55 

Catalysis,  48;  crystallization ,  51,  52 

photochemical ,  51 

substances  useful  in,  52 
Chlorine,  photochemical  induction  of,  48 
Colloidal  gold,  24,  53 
Colloidal  silver,  adsorption  of,  by  silver  halides,  53 

panchromatizing  effect  of,  53,  54,  55 
Crystals,  birefringence  in  silver  halide,  47 

classification  of  silver  bromide,  81,  95,  97 
evolution  of  form  of,  68,  69 

growth  of,  39,  40,  42,  52,  69,  100,  101,  103,  107,  116,  117,  118,  120 
skeletons,  108,  109,  116,  118 

Valeton's  equation  for  energy  in  one  mol  of,  59,  60 
Crystallization  and  chemical  affinity,  50 
at  rest,  39 

Beckenkamp's  theory  of,  50 
defined,  51 
effect  of  dyes  on,  52 
effect  of  mixed  silver  halides  in,  47 
importance  of,  in  emulsion  making,  49 
in  presence  of  colloids,  39 
influence  of  additions  on,  49 
nuclei,  12,  13 
of  silver  bromide,  75 
regulation  of,  50 
Tammann's  theory  of,  28 
von  Weimarn's  theory  of,  28 
Wilderman's  expression  for  second  stage  of,  31 

139 


Cubes,  118 

Degelatinization,  56 

Development  by  ammonia,  after  destruction  of  the  image,  23 
of  latent  image,  18 
of  visible  image,  13 
rate  of,  22 
theory  of,  22 
Disintegration,  Luppo-Cramer's  theory  of,  23 

of  silver  halide  by  light,  12,  83,  84 
Dispersity,  and  the  surface  energy  principle,  57 
and  twinning,  68 
average  degree  of,  70 
composition  as  function  of,  29 
effect  of   colloid  medium  on,  35,  39 
— ,  in  different  emulsions,  28 

—  method  of  mixing  emulsion  on,  33 

-  mixture  of  silver  halides  on,  42,  45 
— ,  on  color  of  photohalides,  53 

— ,  on  sensitivity  of  emulsions,  104 

—  solubilizing  agents  on,  47 

-  viscosity  on,  36 
factors  influencing,  35 
regulation  of,  by  gelatine,  36 
variation  of,  33,  34 

Dyes,  effect  of,  on  crystallization,  52 
sensitizing  action  of,  48,  49 

Emulsions,  birefringence  in  crystals  of,  47 

forms  of  -      —  grains,  93 

mercuric  iodide,  62 

preparation  of,  27 

ripening  of,  by  ammonia,  11 

used  in  crystallographic  study,  76,  77 
Equilibrium,  conditions  of,  43 
false,  43,  44 

of  heterogeneous  substances,  Gibbs'  work  on,  57 
Etching  of  silver  bromide  crystals,  94 
Exposed  plates,  fuming  of,  18 
Exposure  necessary  for  ammonia  development,  18 

Ferrous  oxalate,  ammonia  fuming  with,  11 

Filter  used  in  preparing  photomicrographs,  83 

Fluorescence,  49 

Fogging  after  ammonia  fuming,  14 

Fuming  with  ammonia,  11;  effect  of   moisture  in,  18,  19 

— ,  with  ferrous  oxalate,  1 1 
exposed  plates,  18 
in  absence  of  light,  22 
method  of,  13,  14 
unexposed  layers.  13 
Gelatine,  changes  in  affinity  of,  for  water,  37 

combinations  of,  with  silver  halides,  37,  38,  125,  127 
distribution  of,  in  silver  halide  grains,  125 
double  refraction  of,  128,  129 
effect  of,  on  dispersity,  35,  39 

— ,  on  taking  up  colloidal  silver,  53 
— ,  on  mercuric  iodide,  63 

— ,  on  optical  activity  of  silver  bromide  crystals,  126 

filter  theory  of,  35,  53 
protective  effect  of,  35,  56 

140 


Gibbs'  study  of  heterogeneous  substances,  57 
Growth  of  crystals,  Bowerman's  relay  principle  of,  40 

directions  of  most  rapid,  107,  116,  117,  118,  120 

effect  of  colloid  on,  39 

-  consolute  substances  on,  52 

influence  of  capillarity  on,  42,  57,  101,  103 

suppression  of,  39 
Gypsum,  Hulett's  work  on,  59 

Hemihedrism  of  silver  bromide,  97 
Hulett's  work  on  gypsum,  59 

Image,  destruction  of,  23 

latent,  ammonia-fuming  of,  11,  12,  18 

visible,  ammonia  development  of,  13,  24 
Inertia,  effect  of  ammonia-fuming  on,  11 
Isomorphism  of  silver  halides,  45,  46 

Johnsen's  study  of  sodium  uranyl  acetate,  71 

Latent  image,  ammonia  development  of,  11,  12,  18 
destruction  of,  12 

Mercuric  iodide,  Luppo-Cramer's  study  of,  62 

ripening  of,  62,  63 

transition  temperature  of,  62 
Mercuric  oxide,  Ostwald's  work  on,  59 
Mohr's  salt,  Wulff's  work  on,  58 
Moisture,  effect  of,  in  ammonia-fuming,  18,  19 

Nernst's  theory  of  heterogeneous  chemical  reactions,  31 
Nuclei,  colloidal  gold,  24,  53 

colloidal  silver,  23 

crystallization,  12,  13,  24 

effect  of  gelatine  on,  56 

from  silver  halides,  23 

gelatine  as  filter  against,  35,  53 

ripening,  13 

Octahedra,  86,  100,  106,  108 

directions  of  most  rapid  growth  in,  107,  116,  117 
Opacity,  14 

Optical  anomaly,  69,  122-127 
Ostwald  ripening,  12,  27,  61,  62,  63,  67,  72,^99 
Ostwald's  law  of  stages,  68,  69 

study  of  mercuric  oxide,  59 

Panchromatizing  effect  of  colloidal  silver,  53 
Pentagonal  dodecahedra,  95,  97 
Photohalides,  cause  of  color  of,  53 
Photomicrographs,  preparation  of,  83 
Plates,  exposed,  fuming  of,  18 

unexposed,  fuming  of,  13 

used  in  fuming  experiments,  14 
Precipitation,  of  silver  bromide  crystals,  81,  85 
of  silver  halide  pairs,  44 
von  Weimarn's  theory  of,  30 
Pseudomorphs,  21,  72,  86,  87 

Reaggregation,  initiation  of,  by  nuclei,  22 
Relay  principle  of  crystal  growth,  40 

141 


Reversal,  19,  20,  21,  23 

Ripening,  27;  as  thermodynamic  process,  106 

Bancroft's  theory  of,  56 

Bellach's  study  of,  61 

of  emulsions  by  ammonia,  11 

of  mercuric  iodide,  62 

Ostwald,  12,  27,  61,  62,  63,  67,  99 

Sensitivity,  and  surface  energy,  104,  106 

effect  of  ammonia-fuming  on,  11 
effect  of  gelatine  on,  56 
effect  of  method  of  mixing  emulsion  on,  33 
Sensitizers,  48;  operation  of,  50 

panchromatic,  colloidal  silver  as,  50 
photochemical  conception  of,  48 
Silver,  colloidal,  and  silver  halides,  53 

as  panchromatic  sensitizer,  50 
effect  of  gelatine  on  adsorption  of,  53 
Silver  bromide,  classification  of,  81,  95,  97 
crystal  forms  of,   89-94 
crystallization  of,  75 
crystallographic  investigations  of,  80 
directions  of  most  rapid  growth  of  crystals  of,  107,   116, 

117,  118,  120 
hemihedrism  of,  97 
in  polarized  light,  121 
modifications  of,  97 
plates,  92 

polymorphism  of,  121 

preparation  of,  for  microscopic  study,  81,  85 
sensitivity  and  surface  energy  of,  104 
structural  anomalies  in  crystals  of,  107,  108 
Silver  chloride,  classification  of,  97 
Silver  halides,  and  colloid  silver,  53 

birefringence  in  crystals  of,  47 
capacity  of,  for  solid  solution,  49,  79 
color  of  mixtures  of,  44,  47 
crystalline  form  of  mixtures  of,  79,  80 
double  compounds  of,  with  ammonia,  16,  25,  86,  87 
effect  of  gelatine  on, .35 
effect  of,  on  dispersity,  42 
homogeneity  of  mixtures  of,  44,  46 
miscibility  of,  45,  46 
mixtures  of,  44   45,  46,  97 
photochemical  sensitizing  of,  47 
precipitation  relations  of,  44 
preparation  of,  for  emulsions,  27 
solid  solutions  of,  49,  79 
structure  of,  in  photographic  emulsions,  125 
Thiel's  study  of,  44 
Silver  iodide,  buffer  effect  of,  66 
classification  of,  97 
polymorphism  of,  63 
transition  temperature  of,  79 
Silver  iodo-bromide,  crystalline  form  of,  79 
in  polarized  light,  121 

Sodium  uranyl  acetate,  Johnsen's  study  of,  71 
Solubility  and  surface  tension,  66 
Stability  of  crystal  forms,  67 

142 


Stark's  theory  of  fluorescence,  49 
Structural  anomalies,  107,  108 
Supersaturation,  absolute,  22 

specific,  22 
Surface  tension  and  solubility,  66 

Tammann's  theory  of  crystallization,  28 
Temperature,   effect  of,  on  dispersity,   38 

transition  —       —  of  silver  bromide,  40 
—  of  silver  iodide,  79 

Tetrahedra,  97 

Thiel's  investigation  of  silver  halide  pairs,  44 
Transparency  on  fuming,  14,  15 
Twinning,  68,  69,  70,  71,  104 

Unexposed  plates,  fuming  of,  with  ammonia,  13 

Viscosity,  effect  of,  on  dispersity,  36 
von  Weimarn's  theory,  27  et  seq. 

Wulff 's  study  of  Mohr's  salt,  58 


143 


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